Can I sue my mother, who has reneged on her agreement to pay me?

I was her sole carer for 3.3 years, and I had no income for that time. I wanted to have a relationship with her before it was too late (she has, in the words of her own therapist, failed to be a mother to me), so in Jan. 2014 I agreed to be her live-in aide -- initially for only room and board (about $6K annually), because I assumed I could generate a little income from home as a freelance writer. But by 2015 her needs increased a great deal and by late 2015 they were all-encompassing; finally, in June 2016, I told her that if she wanted me to stay, she needed to pay me retroactively, from Jan. 2014, and that I would charge her only 40% the cost of a live-in aide. She agreed -- but she has been stalling ever since (it's been 20 months).

Despite her stalling, I continued taking care of her -- and continued pressing her to honor her promise; in late summer/autumn 2016, she became unpleasant, she accused me of harming her, and my two sisters installed a live-in aide in March 2017 (the aide does 20% as much work as I did, and she can't handle medical emergencies because she lacks common sense and isn't fluent in English; I have documented how little she does and the dangers she has posed re: not keeping track of Rx, not being present when my mother fell and thus needed to go to the ER).

I have also documented some of what I did when I was the carer,  but I didn't keep a daily log because I was exhausted; is the absence of a daily log plus the absence of a signed employment agreement going to make it impossible for me to sue her? Her therapist knows of our arrangement because I discussed it in June 2016, during one of the three-way meetings (mother, me, therapist), and my mother agreed to the conditions in the presence of the therapist -- but I don't know whether the therapist will agree to speak about this in a courtroom or legally binding setting.

I realize that this situation sounds extreme, so please take my word for it: I'm not the bad guy here. (On several occasions form Jan, 2015 thru Jan. 2017 my mother and I saw her therapist, who tried, as a family therapist, to get my mother to see that she has never treated me as a daughter.) In 2015, for ex., when I needed Rx (lupus-like ailment), my mother refused to pay for it but continued paying for my chronically underemployed 62-y-o sister's cable TV -- yes, you read that right. (My mother has given that daughter about $150k over the past 35 years because she refuses to grow up and get a FT job with benefits.) As noted, my mother has never treated me as a daughter -- sometimes not even as a person -- and I was so starved for a relationship with her before it was too late that I (unwisely) sacrificed my financial and physical well-being in an effort to earn her love. (Yes, I know that a good parent loves freely -- doesn't offer love for sale -- but back in 2014, when I offered myself as the Good Daughter Who Will Do Everything!, I wasn't yet admitting to myself the intensity of my mother's malevolence.)

Anyway, if I were financially able to simply walk away from this horrible situation, I would -- but I can't, because I'm ill, I'm not-very-employable after years away from the workplace, and I'm broke because I've been without a salary (+SocSec contrib+401(k) contrib) for 3.3 years -- because I naively believed that she would keep her promise. So, I need to sue her if I'm going to survive. Any ideas? Thanks.