Breaking a promise although you cut them off your life?

Okay so,
I had this on my mind for a while and I guess I just need opinions or reaussurment, so that I don't completely overthink everything..,
I had this person who I was in love with.
However, I knew she was in love with somebody else so I never got my hopes up or anything, I knew I could never be with her.Actually, it's kind of funny what your first love does to you.Overwhelmed to be feeling such sensational feelings for the first time, I tried so hard to be someone she likes.Could find more important than any other of her friends, a foolish thought, I know now.
Anywho..., although she was in the wrong sometimes, I still apologized and basically took all the blame whenever we had a fight.Now that I've got my pink lovers googles off, I realized what kinda person she was, and how idiotic I feel that I fell for her in the first place.Honestly, I don't even know what attracted me to her.I mean, she's a good person who only means well but the way she delivers it comes off wrong at times.Anyway, I confessed my love for her once but she didn't really get it (she just didn't want our friendship to get ruined through it,) but like I said I never intended to get into a relationship with her, because I knew it wouldn't be possible. But I think she saw it at a different light as in me trying to get with her at every chance possible. Long story short, we ended up having a massive fight over such a ridiculous topic, it is embarrassing to even think about it. But she said some things that really got to me, I felt like the most selfish and biggest asshole in the universe.After I told some of my friends what happened, they were confused - non-biased telling me, that I wasn't overreacting (as she claimed.) and that she was the actual cause of our fight. But nobody knows how painful it is, to realize you care about someone more than they do for you. I cried for days, feeling depressed because that fight ended with us cutting each other out of our lives. It was a "in the moment" kind of thing but then again, it wasn't. It hurt so much whenever she talked about her crush, so maybe it was for the best. But anyway..., you can't force someone to care, you can guilt-trip them but that won't make them stay. I realized that after I thought of all the stupid things I could do to rekindle this whole mess, write a letter, appear infront of her house door, tell her to meet me at the place she likes so much. Honestly,...it really sucked when I saw how she posted so carefree on her social media about her crush and "how much she loves that person with all of her might", she seemed to be more active on it than usual. Of course, there wasn't me anymore to take up any of her time. I know that a post won't show how she actually feels, but I know her. The person she is. Truth be told, we haven't known each other for too long - maybe around 2 years-ish, but still. To feel so tossed away like someone that comes and goes, to feel like she didn't even care hurt more than realizing she'll never be mine.However..., I'm happy now. Quite frankly, I don't want to have anything to do with relationships or love anymore, people only break your heart. But there's this promise I made to her, I don't want to be too precise about it but basically she told me not to read a story she wrote years ago (she said her writing was awful back then) and I kept it although I've been so curious. Now, I know people might think I'm stupid for feeling like this, but I'm trying to be an honest person. And I feel guilty, twisted - she's not in my life anymore and she'll never be again, so does it matter if I break that promise? I know she wouldn't care, but part of me feels like I'd do something wrong since I commit to all my promises.Thank you for taking your time to read the mess that my thoughts are, I'd appreciate some wise advice.
Sincerely.