[Methuselah]

I would never cheat on my girlfriend but it seems you just chose a bad apple for a guy and all I can say is keep fishing and look at the small things he does like for example:

when you are talking face to face will his eyes wander to another woman

(this one depends on how you talk) are his only replays to a conversations with you mostly or lately with "Yes" or "A huh"

does he flirt with other women

Is he too busy to hang out with you more than it seems he should be?

I think you get the point and sort answer to your problems is don't date assholes.