Advice on being paranoid about SO cheating?

I have bad trust issues with pretty much everyone... I've been with my SO for almost three years, we have a one year old daughter together and we've lived together for two years. Last year when our daughter was about six months old I caught him hiding a secret phone that he used to watch porn the second I took a shower or went out... (I don't approve of porn). Since then, he stopped and has been trying to prove I can trust him. However I still get these horrible thoughts (I have an anxiety disorder and I'm bipolar) that he'd cheat on me if the opportunity presented itself... I'm very insecure about myself and honestly dont know what he sees in me. I worry about him cheating if a girl ever came onto him or him flirting with other women... I'm his first for everything (we're 19) and I just worry he'll want to explore his options. He says he only wants me and he can't imagine life without me and that he learned his lesson about hiding things... It's just sometimes he randomly snaps at me and gets so pissy and cold and it makes me worry that he doesn't want me anymore.. advice?