What should I have done differently?

Two words: Energy Vampires. People who suck up the joy and the energy around you for themselves so you feel depleted after an encounter with them. Your mother sounds like an EV who's been playing you like a fiddle for decades now and who you haven't learned how to properly manage.....YET.

I had an EV for a mother myself. She lived to 95 and I dealt with her passive-aggressive hissy fits and histrionics till I was 64. So I had to have VERY FIRM BOUNDARIES INTACT or I'd have lost my mind and the quality of my life decades earlier after she insisted on moving in with me. I said ABSOLUTELY NOT right off the bat and took that card out of the deck. Idk what your living situation is, you did not fill out your profile......so I hope you do not cohabitate.

If so, move out/move her out. The brain damage a mother like this inflicts is mind boggling. They slither into our heads and dwell there like fungus. Which is nearly impossible to get rid of. 😑 Set down some rules you don't break under any circumstances, like when you'll visit, for how long, under what circumstances you'll leave, etc. Don't let HER dictate YOUR peace of mind, in other words. You set the stage for how she behaves based on what you're willing to tolerate. If you get up and leave every time she starts smack talking, she'll understand you mean business. And what you are and are not willing to tolerate.

I love ya ma, I just love ME more so step aside while I show you what that looks like.

Take a peek at this excellent article to help you navigate a passive-aggressive loved one:

https://lifelessons.co/personal-development/covertpassiveaggressivenarcissist/

Best of luck to you.

From your answers to our suggestions, numb, I would say there can be but one answer. Your living with mom isn't working for you. Sounds as tho it hasn't been for some time.

You likely would be much better off--BOTH OF YOU--not living together. I would either move out, or ask your mom to move out. That is to say, whomever is living in the other's home? It's time to move on.

If you need to seek professional help to make that happen, I think you should do so.

EN- Your mother does NOT have a life and she really needs one. She has nothing new going on in her life so she repeats and repeats history.

What does she do all day? Any activities? Friends? Senior center?

I can understand your frustration with your mother's rumination, dwelling on negative things, etc. I agree with others that your mom needs an evaluation to see what's up and what could help her. However, your behavior towards her as you describe it could be frustrating for your mother. You say "After waking I started looking at facebook on my phone and she immediately asked if I was looking at social media, that I needed to stop, that its harmful, etc. I just continued looking at it and she went to the bathroom then came back and literally started sobbing . . ." I personally find it incredibly frustrating when people are looking down at their phones while we are having a conversation or if I am trying to initiate a conversation. Going "grey rock" on someone doesn't necessarily mean failing to acknowledge that they are trying to say something. You can acknowledge that your mom is saying something that is disturbing her without agreeing with her or getting in a long discussion or argument. Easier said than done! I'm afraid.

Grey Rock is not "not answering". I think you need to read more or engage with a therapist to help you figure out how to change this situation.

Seems you have gone a bit "gray rock" on her. Withholding communication can be a lot of things. Training? Punishment? But there is always some sort of hidden message, whether conscious or subconscious.
I myself am wondering why. Are you angry at her?Communication is the way to peace. Coming home from 5 days away the norm would be to talk for perhaps 20 minutes before catching up on media, I would think.

If she says what she said in the car a good response would be: "well, mom, to answer why I am quiet, it's because we have discussed this a few times, and right now I need to concentrate on driving".

It sounds as though when you were away you realized that there is too much communication happening, and you have decided to just stop-cold.

It is sounding like perhaps this situation living together isn't working out for you on a deeper level, and there are things you need to get together in your own head about how long it can go on. Or what changes need to be made to make it better.

You might consider seeing a LSW who does private counseling practice to comb out where things stand on a large scale in your own head. Then consider bringing Mom in to discuss boundaries, limitations, means to better communication and etc.

I sure do wish you good luck. It's hard to live with people. ANY people.

EN, I am not a fan of medicating folks for the sake of quieting them.

What psych meds (specifically, antidepressants and antianxiety agents) did for my mom was to keep her calm and happy, not "drugged" or quiet.

Before she was on meds (if I recalle correctly, it was Lexapro, Remeron and a tiny dose of Klonopin) my mother fretted and worried excessively about the taxes she thought she hadn't paid in 1937, her gay great nephew and and how she was getting to her next cardiology appointment. I thought of her incessant worries about these things as "wearing a groove in her brain" that simply begat more rumination. She was in a nursing home I didn't have to listen to her rumination, so it didn't bother ME but I didn't think it was good for HER to be engaged with things that were of no consequence to her life.

On meds, she was more engaged with staff, family and other residents--she was simply more HER.

I think it behooves you to discuss her rumination with her doc. There are meds that target that specifically.

Your mother is getting on your nerves. After your vacation you realized how nice it was not to be around her for that week and that you really didn’t want to come back to your soul sucking situation.

When my mother tries to bring up the family drama, I tell her I am unable to do anything about it. If she starts crying about it I get up and leave the room.

If she lectures about the evils of social media, tell her you are a grown woman and can do what you want.

Either your mother must live elsewhere or you need to learn to cope better.

Try to do something nice for yourself every day as a reward for getting through the day with her.,

"Respond in a pleasantly neutral way and be done with it."

if it were that simple, OP would have done that already. i think some people here, haven't experienced being with someone who's constantly negative, complaining. you try to bring up something positive, they'll change the topic back to something negative.  

ignoring, being silent, is NOT an act of aggression. sometimes, it's absolutely the right thing to do - morally, and in every other aspect. it all depends on the specific situation.

sometimes talking IS an act of aggression. see? it all depends.

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EN:
as you know, your mother will NEVER change. she was probably negative her whole life. she could have also chosen to be very happy for you, for your vacation, and ask lots of questions about your vacation. instead (totally predictable), she wanted to talk right away - again - about all the negative things, thereby also destroying your holiday mood.

since your mother will never change, there's not much you can do. the only thing left is less contact, or no contact. i think you'll choose less contact. keep in mind, that so long as your mother is alive, she'll keep bugging you with negativity.

your mother (dementia) obviously has a pretty good memory, since she can perfectly well recall all the stuff she's unhappy about. (there are many degrees of dementia, so it's no wonder her memory is still good.)

negative people WANT to dump their negativity on someone. usually the daughter is the target of that dump.

since you live together, and she won't let you have quiet time (that's not by chance. that's because she wants to keep dumping negativity on you, especially if you're trying to have quiet time. she wants to sabotage your quiet time)...the only way, if you continue living together, is to wear ear plugs ("mother, i just discovered i have an ear infection. i need to wear ear plugs for a few days.")...or headphones with music...or headphones for work ("mother, sorry i'm listening to a work call")...

or "mother, we already went over this topic. there's nothing that can be done. you need to accept the situation. let's talk about all the things that ARE in our control. let's go have a fun walk together right now. let's talk about some positive things."
-----this won't work, because she'll continue to dump her negativity. but you can respond anyway with that paragraph, again and again.

I don't think living together is working out for either of you any longer.
Imo , it would be best if your mother was in AL or MC.