Losing Trust in Someone You Love

I love my boyfriend to the death, I just don't trust him. I'm worried that I will never be able to trust him. From the beginning of the relationship we clicked and fell in love with each other fast but he messed up my trust in him by lying and hiding things from me. I didn't want to end it because he meant a lot to me once I found out. Now months later we're still together, and he hasn't done anything (that I know of) and I just can't get over the fact he lied to me and hid things multiple times. Like e-mailing his ex-wife and texting and FaceTiming his ex-gf. That's not exactly cheating in some ppl's eyes, but it really hurt me, and I feel it has the potential to lead to something. It wasn't a one time thing, and his excuse is that he has a hard time letting go of the friendship he had with his ex's. I think it's bullshit and the fifth time I caught him doing it I told him I would leave him if it happened again. We just moved to Colorado and got an apartment together and things have been going almost perfect but I never let my guard down anymore because I don't trust him and that he won't hurt me. He told his family he is going to marry me, and I told him that I would say no (not in front of his family though). I'm so stuck on how to deal with this. I have always been treated like I'm the only girl in the world in my past relationships, now I feel second place to his ex's. Because he knows it hurts me speaking to them but he has done it again and again. Anything he does I'm paranoid about it one of his ex's popping up. I drive myself more crazy checking his ex-gf's Instagram and she is always putting up subliminal messages and tagged him in things before. Anyone been through this or going through this? I just need help and a new perspective maybe on the situation.