Hey Angie, One thing you HAVE to do is stop beating yourself up because you don't feel you're 'supermodel hot' or you aren't good enough. If you weren't good enough, he'd have dated you a few times when ya first met and then moved on. He clearly felt you were 'good enough" so ya still are.
Also "Beauty" truly is skin deep. I'm a decent looking guy, even for my age. My Ex was pretty, but not really 'beautiful' as people tend to think of it. I've dated a number of women who were "prettier" and built better, 'sexier' etc, than my ex. But know what? Many of those women were bores, etc. I fell in love with my wife because she was smart, she was funny, she was pretty, she was all sorts of things I liked and placed greater value on than 'just pretty.'
And while I always found 'getting chicks' pretty easy as a single guy, I did have a strong enough sense of honor that I believed and practiced "Marriage means one woman till death do thee part" Unfortunately, her convictions in that matter seemed to have changed toward the end.
So, if he does have another woman, that is NOT because you aren't pretty or good enough. It's because he's lacking in moral strength, simple as that. Not to say you might not have contributed to that moral slip. If you've been telling him to find someone better and that you aren't pretty, etc., heck you might have actually convinced him.
Just stop doing that. Frankly, it's your computer, so go in and dump all that crap about his social networking. Any E-mails from other women, send them an e-mail telling them to stay the hell away from your husband, just before you remove them from his account and change his E-mail password so he can't see any responses from them. You simply don't have to tolerate that crap.
I wouldn't make a big deal about it. If he rants and raves, just ignore it, wait until he's finished ranting, then matter of factly just say "Sorry" . . .then do it again if he starts back up. He'll get the message eventually.
As for taking his word there is nobody else . . .if a guy goes fishing a lot . . . sometimes he'll actually keep a fish. My wife believably convinced me there wasn't another man, said she wasn't even interested in men, just wanted out of the marriage. Very convincing and believable . . .and a huge lie. Maybe there really isn't . . .but with what you describe, there may be one or more soon.
Just my unprofessional opinions here, so take it with a grain of salt. Many here just think I'm an idiot. Your call, heheh.
Hang in there and know you ARE good enough . . .the fact he married you in the first place proves that.
Life may be short, but . . . well . . . it actually IS short, now that I think about it . . . . particularly when compared to planetary formation and stuff.