My Mom is living with us and I feel trapped, what should I do?

So, my mom's in her late fifties, and I'm 22. Growing up, I was always taking care of my grandparents and my mom. We lived with my grandparents because my grandmother has Alzheimer's and my grandfather was wheelchair bound. Well, now my mom is unemployed, and since my grandfather passed away has nowhere to go. My aunt was letting her live with them, but was threatening to leave her living on the streets. My brother hasn't talked to either of us since my grandfather died, so it fell on my shoulders to invite her to live with us. Here's the thing; I live with my fiancé in a one bedroom apartment, about 800 sqft max, with our cat. It was comfortable with just us, but now it's clearly too small. We don't live in the same city as the rest of my family, so my mom knows no one out here, thus the unemployedness will continue. She's become very slow, has some balance issues, so I'm not even sure she'll be able to find a job, but she's too young for social security and was turned down for disability. I spent my whole young life caring for old people and now I'm back to doing it again. I'm 22, going to work and school full time, and so is my fiancé. We were financially strapped as it was. Not to mention with my mom constantly one door and twenty feet away from us, intimacy has become impossible. I feel like my life is crumbling around me and I don't know what to do. If I drop out of school I'll never be any better than I am now, plus I'll have the debt I've already incurred. I can't just have a young adult life because there's always something I have to take care of, because I'm the good kid. I'm terrified this extra stress is going to kill my relationship and what life I've managed to build for myself since I turned 18 and moved out. I just don't know what to do.