Saw a used condom in my girlfriend's trash basket...?

There isnt a way to force her to say the "truth", first of all you need to also remember that your feelings right now are based on umption, 2nd of all, a relationship without trust is doomed, you dont trust her, your hurting yourself and the relationship, everything she does you question and you end up going crazy over such. The best thing for you to may want to consider is probably taking a break from this relationship.