I don't think he's cheating on you and think the panties are from old girlfriends that he may use as masturbating tools. I know that even that may disturb you, but I don't know if it's something to get too worked up about at this point if your relationship is otherwise great. For example, I love my boyfriend and would never cheat on him but I sometimes fantasize about other guys (mostly strangers but sometimes an old boyfriend or two) if I masturbate (infrequently and mostly to relive PMS).

Please don't use this as an excuse to go on a witch hunt or to start the squirrels running on that wheel in your head. I have the same fears that you do (never been cheated on by a boyfriend that I know about, but father had a mistress for years) and it can be a quick "go to" place in my head...mostly with little or no cause.

I do hope he understands your fears and is open to you talking about them but try not to accuse him of anything without proof...if he does cheat on you, that's his fault and he's an ***. If you push him away because you're afraid and he's done nothing wrong, then those doubts you're fighting will win. I think the latter is a much worse thing in the long run.