How do I conquer sexual sin between my girlfriend and I

I am 20 years old and have been dating my girlfriend for 3 years now. We started dating junior year of high school and were high school sweethearts. A year into dating, we had sex for the first time and ever since then we’ve struggled with it. We have set boundaries, and reset boundaries again and again. Sometimes we do well, and other times we go all the way multiple times a month. As of recent, I am facing huge conviction. I want to honor God with my life above all else. I find myself hiding from God and going backwards in my walk with him after committing this sin. I cannot win against this sin, and I know the Bible says to flee from it. I also do not see spiritual fruit in her life and I think she has changed a lot since she started going to college. I know the Bible says it’s better to get married than to burn with passion, but I’m so torn on what is right. I love her so deeply and don’t want to do wrong by her by breaking up with her after 3 years.