Verge of a break up over something so small

I've been with my girlfriend for 11 months and for the most part we're happy. We basically live together. Rarely got sick of each other. Genuinely clicked on lots of levels. I felt like I was dating my best friend. But then there's this whole other side of things. She's severely depressed. She has been since I met her. She just started seeking therapy at her school (college) and isn't really making it a priority. I've dealt with depression on my own as well, and have sought out therapy and for a while I was really good. I'm not blaming her and try not to, but her negativity really has affected me. It's contagious. I always thought that since I've had experience with it, I could help her with hers but I can't. I can be there for her, but I can't REALLY help her. Only if she wants to change will she. But at the end of the day she isn't doing enough about it and it is affecting me and causing me to feel down as well. She's even told me she has thought about, suicide from time to time. (that's when I finally got her to call a suicide hotline and her school therapist) What I'm trying to say is, that it's been as tough as it has been good. So back to the story, other than all the depression stuff, we're happy together like I said. But yesterday, I was at her place and she was on her laptop IMing someone. It was open and we were together so I just looked at what she was doing on her screen and on the side it has other people she has IMid and like a little snippit of the convo. This guy named TONY said he was on his way to get her a day or two ago. As weird as this may sound this Tony guy and my girlfriend are all over liking each other pictures. WHICH ISN'T MY PONT! Really it isn't. We had just talked about this before because she saw that I liked a girls picture on instagram and she didnt like that, which, is fair, so I mentioned this Tony guy and we both agreed not to do that while we're together. Not to sound 12, but really, thats the story of Tony. So I ask her what that message is. She's like "Oh, Tony and I hungout" with this "just got caught" face. So I'm pretty pissed, not just because it's Tony but because she didn't tell me at all. It's something a little kid would do to avoid something. She usually tells me anything even if it's a hard thing to do. So she didn't tell me and I was really pretty mad. She told me he has a girlfriend and they all had coffee together, but why should I believe that? If she didn't tell me they even hungout, why would she tell me if they did something else? She hasn't told me any lies before, or kept anything from me that I know of. But now it's like even if a little trust is gone, it feels like a lot. So we were fighting after I saw it because I was mad she didn't tell me and she said she didn't do anything. It may seem petty, but to me, not telling me something like that makes me think that when somethings hard you just wont tell me, when it's something bigger. It was a red flag. So I left her apartment mad, which really upset her. But I feel like because she's upset, I need to just dismiss my problem of her keeping this from me, and focus on her. I get no help. This is where I get in my head. Am I just being an asshole here? Her sister called me(they live together) to tell me she was mad at me for leaving. But why shouldn't I be able to leave? I know problems should be dealt with head on but I was mad and wanted space and time to think. I feel like with her depression all this focus is on her all the time and even if I feel right, it's made to seem like I'm being petty or always have a problem with something. Her sister told me all the same things my girlfriend did, telling me they went out for coffee, but I told her that that wasn't my point. My girlfriend didn't tell me and it was on purpose. So my girlfriend calls me later that night and starts saying stuff like she feels shitty and didn't do anything wrong and doesn't deserve to feel like that. I agreed. I agreed she doesn't deserve to feel like that and that if I was really the one to do that to her, maybe this isn't working. She said she couldn't do it anymore and I told her I'll bring her stuff over tomorrow and get mine from her place. Because of how sudden this was, I'm just second guessing myself. Do I have a leg to stand on here? Or am I taking it too far? It would be a lot easier if I didn't love her so much. But them's the risks, I suppose. -Louis

I'd like to argue constructively but she just doesn't want to consider that she did anything wrong. We don't HAVE to break up but it's been about a year and something like this make me worried about investing more time into someone who doesn't respect me enough to be honest with me. I was more wondering if I'm actually overreacting or if it's okay I feel kinda shafted that she kind of went behind my back. This isn't our first fight so don't assume the break up is a go to thing. Thanks for the input!

I appreciate your input JAN it's probably true and I'm just realizing it. POP: I hear what you're trying to say but you're judging a lot just by misinterpreting the title of my post. And you're being an assh*le about it. This is a place for help. And if you think that's what you gave me, I don't want yours.

I mean maybe you're right, Pop. I don't know and that's why I asked. It's not as clear to me as it seems to be to you. That was the whole point of my post.

It was sudden to me. You can't help who you fall in love with and once I was hooked, I was hooked. I didn't want to not be in love. Maybe that's foolish to you, but it felt good. I sought out help from strangers because I wanted some. I didn't want some person on the internet taking their frustrations out on me. There's no reason to be such a blatant assh*le about things and THEN say good luck. That's a prick job, and douchey. So kindly f*ck off, if this is what the website is about I'm pretty disappointed.

You are absolutely right Louis. If she did it today behind your back, tomorrow she can do something more worse. And the funny part is she just doesn't seem to get what the issue really is! Looking with love in your eyes you might feel meeting Tony is not the issue but it is just as big an issue. She says Tony and I hung out first. Then says his girlfriend was with him later on when you get annoyed. Isn't there another red flag there? I feel its best to move on here. She just doesn't look to be made for you.