If you are feeling depressed and lonely while WITH her, then what's the point in being in a relationship anyways? Being in a relationship should have more benefits than hardships. That should be true to everything you do, otherwise it's all worthless.
Guy, she cheated on you once. I don't know how you let her by that. Then she cheated on you again, I don't see what's the point. Look at it this way. Would you ever cheat on her?
By assuming you've came to this site wanting help because she's done bad things, I'd think not.
Why are you letting this girl treat you worse than you would ever treat her?
Do not date someone who doesn't care for you. It's just a waste of time. While you are like the crying wife who sits home and waits for her husband to come home from his affair at work, she's out getting it on and having fun. FIGURE IT OUT.
Drop her, go meet new people. I promise you that if you do, you'll most likely end up in a much better relationship. Don't lower your standards. You are the man. No woman should treat you badly. If she does, then you move on, and that's the end of it. If she was a guy and she constantly lied to you and messed with you, would you put up with it?
FUCK NO.
She's treating you worse than a male friend would.
Ditch this bitch and move on. I know it's tough to get it to heart, but get it in your mind and just DO IT. If you're this involved, the next few weeks will be killer. Ignore her, forget about her, meet new people, BE HAPPY. Enjoy the things you like to do, rather than wonder about what she's doing.
If she cheats and you let her get away with it, what makes you think she wouldn't do it again?