I [19M] am considering breaking up with my [18F] girlfriend of over a year and a half because I?ve fallen out of love and have feelings for someone else. How can I approach this in the least hurtful and harmful way possible?

Before I start, I realize right off the bat that a) I?m not at all the good guy in this situation, b) there is no way in which this WON?T hurt her, and c) I don?t expect her to still like me or want to be friends with me. I just want to figure out how to do this in a way where she gets hurt the least, because she doesn?t deserve that
TL;DR at the bottom
I met my gf when we were both sophomores, and we started dating our junior year. We recently graduated. For the first year and several months, things were amazing and we felt in love. I was na?ve. I know that highschool relationships rarely last
About 3 and a half months ago, my gf started having second thoughts about our relationship, and began to close herself off and distance herself from me. This was a very difficult time for me, as I am a very affectionate person and I need that in a relationship. Over time, without realizing, I began to fantasize and crush on other people, specifically someone that I became friends with at the end of my junior year
Now, a month into summer, I realize that I have really fallen for this new girl, and no longer feel the same about my gf. About a week ago I had a serious talk about how things need to change in our relationship if it is to continue, and my gf understood, apologizing for how she?d acted and the things she?d done (the specifics of what she did aren?t important here - only that it was toxic behavior and she?s apologized profusely)
The first two or so days afterwards, things were great, but after that I noticed that she had already started moving backwards into old behaviors. My patience is thin and my entire friend group agrees that I should break up with her. Last night, my gf came over, and I didn?t enjoy myself near as much as I used to. I simply didn?t have fun. To me, this is the nail in the coffin. I plan on breaking up with her next week, if not this one
All that being said, I truly believe that she?s not a bad person. She?s just done things that I can?t get over, and continues to do some of them. We?re both young and have plenty of time to grow and develop and mature. We simply weren?t ready for a serious relationship. She doesn?t deserve to be hurt, so I just wanna know how I can break things off in the least hurtful way possible
TL;DR - I?ve fallen for another girl after my gf closed herself off from me for months. Despite promises to change, my gf seems to be the same, so I want to end things. How can I do this without hurting her too much?


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