Short term memory loss and cognitive connections loss. How do I handle all these changes?

I think this will not get better, and I think that you know that. You are seeing the decline that we ALL see. And you know where it is going. And you are seeing it could consume your entire life. And it could.
You are going to have to accept your limitations. Explore that with your on line person.
You can't cure this. You can't fix this. This won't get better. And you have expressed it so well. So I know you see and understand this.
So. Your limitations. Know what they are. Start to journal. Start to decide "when". Some men aren't talkers; if yours is, tell your concerns, and try to come together on what will make you have to consider placement. It could be years off. You don't know how fast things can progress, or how slowly. So you cannot guess. But when you are there more than you are with your partner? You know then you are losing your life quite literally.
Discuss things with your Mom if she is able. If she is not, don't whisper a word. The former is the more likely. Come together with your partner with a plan. Who will be POA, what circumstances indicate that placement is necessary. Be willing to talk about it. If you have to set a time "30 minutes every Friday evening with a glass of wine" then do that.
I trust you will know. Do NOT expect you can fix things. Not everything can be fixed. Do not expect things can be happy all the time. Mom is starting a decline. Things will often NOT be happy. Do not expect to be a Sainted Martyr. They always get little thanks and are expected through eternity to answer the prayers of us all.
I wish you so much luck. I think you know what you are seeing, what you are feeling, and you are trying to plan ahead. That is great. But also know that some monkey wrench is gonna get thrown at you any second. Be ready to duck. You have my heart. Keep updating us.
Do what you can to set things in motion for a clear POA, or DPOA for financial and health care. Discuss with Mom what she wants if she passes out and "goes" on you. Get together with her and her Doc. Try to make a solid base on which to flounder about (as we all do). I FEEL you trying to do everything right. Know that, in this, some will be more wrong than you could ever imagine; that's OK. Do what you can. Stick around the forum and read. Wishing you all the luck in the world going forward.

I am an only child and see lots of changes in my Mom as well. From her anger towards me when I try to address her lack of safety, to bad decision making, to disparaging ethnic groups (includes her son-in-law) to inappropriately touching others.

You are retired - congratulations on having more time for yourself! Please schedule in daily time for self care - reading, walking (or other form of physical activity), enjoying nature, a hobby, etc. As lealonnie1 said- you will handle issues that arise with Mom as they come. No need to worry ahead of time ...

This online support group is available daily and has certainly helped me. Search around for a caregiver support group by contacting local hospitals.

Wishing you all the best. Post us on your progress.

I'm an only child too, so I know the anxiety of trying to be 'everything' to our mothers. You should probably look into getting her some in-home help rather than living with her part-time, because you need a life of your own, too. You're not going to be able to take away her pain, and you can't internalize it YOURSELF, or you'll wind up suffering health problems as a result. Your mother has lived a life of 81 years already, and now it's your turn to live your own life. Help her as much as you can, of course, but not to where you're making yourself sick. "Worrying" all the time isn't going to help you, either.....it's just ruining your joy TODAY expecting disaster to strike TOMORROW. You'll deal with the troubles if and when they come up, and not beforehand. That's a good message to repeat to yourself on a daily basis. Live the "One Day at a Time" lifestyle, for your own sake.

Take care of YOU, my friend. All the best.