Making up for mistakes

OK I was sexting this girl my gf found out. At the time we were engaged and she called off the engagement but stayed with me. I've tried making up for it but every fight we have had since then goes back to that one Fuck up. I know I made a mistake and have tried everything to make it up to her. I gave her a hallpass which she tried to use but never went through with it. Now she found out October/november last year. Idk what else I can do to prove I love her and move on from the mistake