You seem to lack any appreciation for how inaccurate gossip can be.

Just because she has no reason to lie that you know of doesn't mean she is not lying. Just because she actually isn't lying, doesn't mean the information is true. The fact is you don't know and you never will unless you drop the teenager logic and attitude and approach this like an adult with T, including understanding that what you heard may not be true. Or just cancel with her and get another T.

The real issue to me is your lack of proper boundaries with people you are close to. You should have never snooped through your partner's private Papers and the fact that you see nothing wrong with it is a huge problem. You should also not have snooped in your T's private business by going out looking for gossip about her. I think the reason you are not willing to raise it with her is because you don't want to admit how you found out, which is quite similar to how you found out about your partner's affair.

You would probably be better served if you explored the reasons for your own poor relational behavior rather than focusing on how bad your T is, when you don't even know for sure. Whether you do it with this T or not, you are the one who needs to look at herself.