My jealousy is ruining my relationship

I've been with my fiance for about a year and a half now, we have not long had a daughter together but I find myself driving myself crazy over his past relationships. When we got together, there were a lot of issues with his ex who made things extremely hard for us but we fought through it. Since having our daughter, my confidence has completely gone and I feel so low about myself. My body has changed and I look in the mirror and just cry. Theres not one thing about my appearance I'm happy about. I now find myself constantly comparing myself to his exes or women hes had sex with and I'm driving myself insane. Making up scenarios in my head and questioning everything. Hes lied a fair bit in the past and tbh I dont trust him but we are working on getting that trust back, although its bloody hard. How can I gain my confidence back and stop stressing about his past? I know ultimately it will end up ruining our relationship but I cant seem to snap out of it. Please help.