Childish Girlfriend? Let her go?

Hello everyone...I'm sorry this is so long, but this is kinda my last hope. I really need sincere help with my dilemma. I’m 21 and have been with my girlfriend for 2.5 years now. I shamefully say that I can’t tell you how I feel about her anymore. If you were to ask me about a year and a half ago, I would have gladly told you how much I loved her and how much she means to me. But now it’s different. After 2.5 years I still cant “read” my girlfriend. I NEVER know how she’s going to react to something (big turnoff for me). I consider myself to be a mellow-tempered, down-to-earth guy who doesn’t really let things bother him. My girlfriend however LET’S everything bother her. We argue constantly. I will honestly say that I start about 1.7% of our arguments. And the things we argue about are literally meaningless. I don’t know what to do anymore. Just last night we were having a perfectly enjoyable conversation on the phone and all of the sudden BAM! we’re arguing again. (when I say “we argue” I mean she squabbles about something and I take the brunt of it. It’s like this regularly, and has been for the past year and a half. She says she loves me and that she wants to marry me but I don’t see how if she’s never happy. It’s not just me neither…she has these problems with her brother and her mom too. But I can never confront her about it. She ALWAYS tries to turn it around on me like I’m the bad guy! She’s the best con artist in the world. “You never understand how I feel!”… “You’re such a jerk!”… I don’t believe in using the excuse “I’m a guy and that’s how guys are”, but I do believe that guys and girls think differently. All my emotions shutdown when she starts arguing over pointless things… I’ve never had anyone make me as mad as she has, but then I’ve never had anyone make me as happy either. A couple of weeks ago we agreed to take a break for however long it takes. She has expressed in the past several times that she’s wanted to take a break. She was calling two days later saying she wanted to call the break off. I don’t understand.
She really pressures me about marriage also. I don’t think we are near being ready for marriage! I’m not even sure is she’s the one I want to marry anymore. Somehow she doesn’t want what’s best for us in the longrun, she wants what’s best for her right now. I just think she wants to be married, not be married to me. How do I tell her this? What should I do? Not to be ugly, but does she have a chemical imbalance in her brain?
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