Girlfriend cheated on me, one year later still think about it

So this is the story, my girlfriend and I were 3 months together, we went on vacation seperately in the same week,
so one day of my vacation she texts me if she can tell me someting, i said yes
and she said she kissed with a boy and she didn't want to lose me,
So ofcourse i was really angry and upset, but i still wanted to stay with her so as the vacation continued, i was sad every day and tried to feel better by just drinking it away, then after vacation i saw her again, we talked and it was kinda okay again.

I tought about it a lot, which is logical i guess, then like 2 months after it happened, she was upset and i asked what's up, (i didn't think about it anymore, or ye not much, because it was just one kiss)
then she told me i love you so much and i need to tell you something blablabla, she told me that he actually stayed at her hotel room that night, because he was locked out of his hotel he asked if he could sleep at her place, she told me nothing happened, she slept with her clothes on and went to sleep immideately when she arrived, ofcourse i felt betrayed again, and the thinking about it started again, every single day i was sad, slowly but surely it was disappearing, until one day i decided to contact this boy
(i was 17 she was 16 the day it happened) so i asked him what really happened
he said i'm not gonna tell you anything, after a while he said that they kissed multiple times the next few days,
so i called my girlfriend to ask if this was true, she said no, i asked again and she started crying that it was true, apparantly they kissed again the next day (when sober, the first time was when she was completely drunk) so i was furious again, after 2 times lying she still didn't say the truth,
i asked is this everything and she said yes it is

Then ofcourse the day after i figured another think out, she apparantly sucked this guys dick that night he spend the night with her, i asked her if it was true she said no ofcourse not, i kept on asking it and she said i was completely drunk i don't remember anything from that night,
and she asked her female friend that also stayed in her room that night if it happened, and she said yes, she was ashamed and i felt betrayed for the 4th time

I was so dumb to stay with her because i believed her she actuall ydidn't remember anything, then one night i just asked her reall ynice, be honest with me, did you really not remember anything, she confessed and said yes i do remember it... so i felt betrayed for the 6th time...

Now we are almost 1 year later (the last thing i figured out about she remembering it was like half a year ago now) i still think about it frequently , and on the way every time i tought about it and was sad, i wanted her to show me she felt sorry and talk to me, but every time she became angry and never talked to me anymroe, saying "i should leave the past behind and move on", so every time i needed someone, she wasn't there for me,

Now recently she said to me: promise me you'll never talk about it again, this is affecting our relationship, if you won't let it behind, it won't work out. I don't want a relationship with this kind of tension the entire time..

But i just think about it regulary and most of the time i keep it quiet because i don't want her to get mad again or feel bad about herself because i know she feels really bad when we talk about it because she has so many remorse.

It's getting less and less the thinking about it.. but like i still think about it like almost every day, sometimes i don't thingk about it for a week, but most likely daily.
I really need advice