Insecurities are ruining my relationship??

Chelcie • 28. Mum to 2 girls 🫶🏼 UK
I need help. Me and my boyfriend/ex (whatever we are now) have been on and off since August last year. 
I've always been pretty insecure about myself, as I've struggled with my weight. Knowing his ex was a size 6, beautiful blonde, with fake boobs that did modelling.. Makes me insecure. He tells me he loves my body and all that.. But I can't love myself. If he touches me I flinch. 
The other day he tried to take my shirt off.. I froze and wouldn't let him. 
I want to be able to have "sexy time" and for it to be enjoyable. How can I get over my insecurities!? I'm trying to gym it.. And have lost a few lbs. When we first met I was around 14stone. I went down to 12.3 and now I'm 13.2 please help 😩😩 I don't want to hold him back for all these desires he has 🙁