<human>: I'll try to keep this short, but I have caught my girlfriend of almost two years lying to me. She doesn't know I have seen proof she's lying, but I brought up the situation to talk about it and see what she said. We spoke about it for a very long time and I gave her many opportunities to admit it, but she didn't. She even then lied further saying the issue wasn't happening. 

I know this post is kind of garbage because I can't go into too many details as she is an avid redditer, but I need advice. I don't know what to do. It's not the biggest issue in the world, but big enough that I can't stop thinking about it. (Round about explanation, I think she might think it's not a big deal and is concealing it to avoid hurting me over something she thinks is nothing.)

But the fact that she lies makes me feel like I can't trust her. The biggest problem is I can't tell her how I got the information, so it's hard to directly confront her. I'm afraid knowing is going to completely kill my feelings for her as my ability to trust her is damaged. I love her very much, so it's hard to even think about confronting her with the evidence.

I feel like I risk destroying the relationship if I confront her, yet think knowing is destroying it as well, only from inside me. 

Has anyone gone through this? If so, what did you do and how did it turn out? I'm not willing to be lied to significantly like this, but also not very interested in blowing up the relationship. 

Help?

Condense the text into a few key points
<bot>: I asked her about a situation that I had proof was a certain way, and she said it wasn't. Even made something up instead.