Can't trust my boyfriend, fear he will cheat on me? Please help?

[W]

This is more of MY own trust issue than it is my boyfriend's fault but I just CANNOT trust him whatsoever! I want to so badly. He has a lot of female friends, especially on facebook, he tends to "creep" girls profiles a lot, which I don't like but whatever. He has a bit of a flirty personality which worries me. He's never gone behind my back or cheated on me though. He has gone behind my back once actually and hung out with a girl who he had slept with in the past. He didn't tell her he had a girlfriend and they hung out, didn't do anything. I found out by snooping his phone. The text messages weren't flirty at all. However he said he got the impression, from hanging out with her, that she liked him, and he didn't want to hurt her feelings so that's why he didn't tell her about me. After I found out, he texted her and told her he has a girlfriend and he can no longer hang out with her. To my knowledge, that was the first time they'd seen each other in over 3 years, they're old friends. That happened over 8 months ago and it still upsets me!

I constantly snoop his phone, facebook, computer history, everything to look for any reason to lash out at him. I feel completely psycho I haven't snooped his stuff in a while, but I fear that he hides things from me and I'm just looking for any indication that he's being unfaithful or flirty with other girls to confirm my suspicions. He's loyal to me, crazy about me, and very sweet to me.I just want to make sure I'm making the right decision in wanting to spend the rest of my life with him, Im scared he will cheat on me in the future or I'll find some flirty text messages and that'll be it. I don't want this to end in divorce, at some point in the future because we plan on being together for the rest of our lives. He does everything he can to make me trust him and I still can't.

Help
We're in our early/mid-20s, please mature answers only!