How do I stop being so stuck up on the past and how do I stop being negative about myself?

I am 20(F) with a lot of bad trauma that I can't seem to get rid of. Growing up, I was so plagued with expectations. I always lived up to people's expectations of me, and over time, I just started having this major decline. From my academics, my mental health, and to my whole social life in general. It was all bad, response from people around me were all bad. It was normal for me to believe that I was lacking in all aspects after hearing it from multiple people. It's like it has become programmed in me to resort to listening and believing the hurtful words I've received. There were times were I got out of that state and dated, but I can't help but notice that I have a recurring problem with my confidence. I suspect this issue came from getting told by an ex that me in makeup was basically like "putting lipstick on a pig" (I was and still am quite chubby and it stuck with me.) There are times where I'd feel pretty to the point I'd delude myself that I look slimmer than I actually look when I'm not. And there are times where I'd just be so scared to even look at the mirror, because my very own reflection was enough to make me throw up. Aside from that, I got unwanted attention from an old man and was assaulted. I didn't even realize until I grew older. Slowly, I unintentionally ruined myself. I started to snack when I'm stressed and even went as far as doing self harm. Anything related to intimacy or my appearance scared me. I have snapped out of it though. I want to work on myself. Right now, I have a boyfriend and he's been the best. I appreciate everything he's done for me. Although, I cant help but feel bad for him because he had to hold back lots for my sake. I want to lose weight and look pretty for him, but it hasn't been easy. I feel so guilty, because I feel like I'm fooling him in a way. He recently brought up wanting to do a photoshoot with me and he seemed so happy. He mentioned about buying me a dress and taking pictures somewhere pretty. The thought was nice and cute, yes, but I felt absolutely terrified at the idea of someone else buying me clothes. I have been struggling to look at myself in a good light or even feel pretty in my own skin, I don't know how I can do a photoshoot in this ugly state of mine. I've been wanting him to give up on me and the relationship, but I love him too much to do that, and he, himself, always replies that he'll stay because he wants to help me just as I helped him. I want to be good for him but with my current state, I feel repulsed. I feel like I'm promising him so much for a future I can't even provide. I have been trying to not be overly self critical, but there are multiple instances where I'd just realize that I was doing it unconsciously again. I'm scared it will inevitably lead to me and our relationship's ruin. How do I get myself out of this mindset?