In need of major relationship advice. (Sorry for the length!!)

So my girlfriend and I have been on and off for a good 2 years now in a long distance relationship, but not far enough to where we can never meet. We’ve met in person many times and they were the best days of my life. To start it off, my girlfriend and I have a very mutual feeling that the both of us know each other like nobody else could ever know or understand us. We’re extremely connected in ways I can’t explain and we’ve been through so much together, hence why we’ve been on and off, we hurt each other due to being young and immature but our love always prevailed and we grew from it. 
More to the point of this question, last summer I had an extremely rough summer and I was dealing with a ton of things. I’ve always been a person to suppress my problems in order to be there for others, and this time she really needed me to be there for her, and truthfully, I was not. She sent me many messages of what she was dealing with and I was not there for her in the way I should have been. We happened to both be going through a rough patch that same week and it was a week where I could not bring myself out of my own head and suppress anything. Although our communication skills have definitely gotten better over time, I spent that week trying to make the most supportive message/letter for her instead of just being there for her in the moment. I made her wait for a response and that really hurt her emotionally, and it was just extremely shitty of me. I was devastated at what I had done and still feel guilty about it to this day, considering that she recently brought it up to me a couple weeks after getting back together in the summer. Fast forward to october when we’re both in college, two different colleges, and it really took a toll on us to the point where we had a falling out. We barely talked at all which made us super detached. She winded up calling me one night around 3am but I winded up not hearing the call, and clicking unavailable on the other call because I was asleep and groggy and not thinking straight. I didn’t think anything serious of it in the moment because we hadn’t called or talked properly for about two weeks. I guess it just detached my mind but it was also something that hurt her emotionally considering she needed me and I wasn’t there in the moment for her.
I just wanted to state that there were many times where I was there for her, staying up late into the night and refusing to sleep until she was better, it was my number one priority and I let that slip twice which really devastated her and it broke my heart immensely to know I hurt the person I loved the most. I guess the part that most confuses me is how after our falling out, we got back to talking and realized we didn’t want to be with anyone but each other. We started dating again and she told me she was the happiest she’d ever been knowing we were back together. I learned from everything and the pain I caused her in the past, and she learned from the pain she caused me. I was willing to do the work and try and again, I went to counseling because i was so distraught and really tried to sort myself out to become better for her, even though I always cared, sometimes my actions didn’t show that. We got back together a month ago and since then I’ve really been there for her through her hard times at college. There were many times she called me in distress and I was there for her as long as she needed me to be and I’ve really been there for when she couldn’t even be there for herself. She went back to college and boom, I think it crashed and burned again. She brought up the situation from the summer and the phone cal situation out of nowhere, called me heartless, and started comparing me to other people. Part of me thought I deserved that title, but a part of me also did not. I”ve always been one to help others and put my needs aside and the one time I did do that, it was when someone else needed me and i still cannot forgive myself. I just don’t understand because she was so happy to be back together and then she just started bringing back up all these past happenings, and she doesn’t even acknowledge the things she did. Only me. She calls me heartless, and yet I’m willing to let the things she did to me slip so that I can focus on becoming a better person and focusing on her feelings. I just don’t know what to do in the situation and she will hardly communicate with me at all and refuses to call me, which to me, is the adult thing to do. She tells me she doesn't have time to call me but will text me long messages. Part of me feels like she's right and that I deserve al the words she's throwing at me considering I've been feeling the guilt and heartbreak of causing her pain for months. I wanted to focus on strengthening our relationship and transforming all of the bad things instead of focusing on hurt in a negative way. But part of me also feels like she's just attacking my character. She'll tell me how lucky she is to have a person like me who gives her so much support but as she soon as she went back to college, it's like her mindset completely flipped and I cannot handle losing her again. It was the hardest months of my life, but I also know I caused her pain and that her feelings are completely valid. I can not see myself with anybody else, but it isn't even that. I'd do anything for her and that feeling has only gotten stronger over the year, as does my love for her, and I just want things to be healed and fixed but I know it doesn't always work that way. i guess I just need some advice. (Apologies for the length and I appreciate anyone who takes the time to reply.)