Advice desperately needed please!!

So I've been with my partner for a year and a half now and we have a 1 month baby together we've arguing a lot lately as I always think he's checking out girls when we're in public and I constantly think he's looking at girls on his phone I have extremely low self esteem and pretty much think I'm the ugliest thing ever and always compare myself to other women in the first 7 month of our relationship he would go on dating apps, flirt with women online, check women out and like certain things online all of this has made my insecurities 10x worse as I don't look like these women. For about almost a year he's tried his best and has stopped everything and really does try his best but I'm so paranoid and think why would he want someone like me he was scrolling on Instagram and had a good look at a post of a pornstar on the best that came up on his news feed we've had a huge argument about it how do I change how do I trust him, gain self confidence and let the past go Please 🙏 anything suggestions