Idk what to do

The long story short, me and this girl have been together for a little over 3 years, but before the 2 year mark everything kinda fell apart. I was going through personal stuff at the time and I wasnt able to see her very often, and she was very stressed with school and emotionally she had her own issues. During that time period, she made a friend at school that was helping her with a class that I wasnt there to be able to help her with and she ended up developing feelings for him. Eventually that led to her cheating on me and we broke up. Fast forward a 4 months later and she genuinely seems like she regretted her actions and asks for another chance. I still had feelings for her and always wondered what if things could have worked out for us, so I chose to give her another chance. Since then we've been able to work things out and we're still together over a year after that. The issue now is that I still cant get the thought of her cheating on me out of my head and its really starting to take a toll on me mentally. I dont know what to do because I still love this girl, but I'm not "in love" with her, if that makes any sense. Every time I want to love this girl the way I used to, I always flash back to her cheating. If i leave, I walk away from my best friend and theres no guarantee that I get better, but if I stay I think I'm going to be stuck like this. Idk how to bring this up to her without her thinking that its over for us. I'd rather try and get better than throw this away.