Girlfriend contemplates breaking up - where did this come from?

My girlfriend and I nearly broke up at the beginning of the summer that just passed. The reason being was we had been miserable together for the past couple months prior to that. She’s an incredibly emotional person who requires a lot of emotional support, but is not equipped to give it back. So, when I’m happy with life, but she’s not, the relationship is still good. However, whenever we’re both unhappy, we’re like two drowning swimmers hopelessly trying to support each other. Even worst, when I’m unhappy and she’s happy, she soon becomes unhappy because she doesn’t know how to be supportive (which makes her feel inadequate as a girlfriend), and she gets all anxious/nervous/super-sensitive because she gets paranoid that she’s the source of my unhappiness despite telling her 1023180 times that she’s not. So, when I’m unhappy and she’s happy, she soon becomes unhappy too, and somehow by the end of the day it turns out that I’m having to console her…

Anyways, we talked things out last summer, agreed on a way in which we had to deal with our issues, and it worked – we had a great summer. Just last week we went apple-picking, and she was ecstatic; she just kept telling me how happy she had been for the past couple months. Just six days ago, she gave me a promise ring on my birthday (I had given one on hers).

[ I should mention here that we’ve been dating for a shade less than 3 years, and we’re both 20. ]

This week, however, for reasons not necessary to state in here, I have been very unhappy (for reasons having nothing to do with her), and very lonely. Over the course of the week, she only saw me for a few hours one night. I told her a number of times how I was feeling unhappy and incredibly lonely, but it was completely wasted upon her. She packed her schedule full of extra-curricular things she didn’t have to go to (as in even when she scheduled them, she was not obligated to go in any way), and never even made an effort to see me. Yet, somehow, when the tables are turned, I’m expected to be there for her… Note also that she’s unhappy and requires support much, much more than I do.

I voice my discontent to her about her being completely emotionally unavailable in a way which was neither accusing, nor aggressive, nor whatever. She freaked, said that I was making her feel guilty for wanting to do things, and then she panicked and said she feared that the misery of last school year would return and then seriously started to consider breaking up with me.

wtf. Last summer, when things were terrible, we discussed the approach we would need to take to address conflict and resolve issues rather than let them resurface later. In the face of the smallest ounce of tension, she completely abandoned it and considered abandoning me and reverted to this old irrationality and simple-minded stupidity which got us into horrible situations in the first place. Given the great summer, the promise ring, where the fuck did this come from? And more importantly, what kind of person is this girl? After having such a great time for the past couple months, after everything we’ve been through and all the thought and discussion we put into engineering a way to get through our troubles (which WORKED when we used it; it was the reason for our great summer), how is it she seriously considered just leaving me in response to pretty much nothing despite how much she claims to love me (claims which I believe are genuine)?

Soon after, she abandoned the idea of breaking up, and apologized a number of times for the things she puts me through, but I’m still angry and I don’t know what to do. In one fell swoop, she’s made me feel more alone than ever, as well as insecure, disposable, and worthless. Not only that, it feels as though she’s effectively put us back right where we used to be.

Now, I find myself angry and resentful with her, despite how much I love her and how much I want to spend my life with her. It’s only with this girl that I have ever felt this alone in my life. Never before with anyone else has someone made me wish I was dead more than her… and I think this is not because she’s a bad person (despite being selfish and borderline delusional when it comes to topics dealing with emotion), but because I genuinely love her for the many positive things that have not been written here.

Where did this come from? What should I do? How would I cope with the solutions available to me?

Any advice or insight is welcomed.