Is this cheating and is it fixable?

I know what I did was completely my fault and I feel incredibly guilty but I want some insight from others about the situation. For context, I told my girlfriend that I had briefly masturbated to another girls photos on instagram. This wasn’t just any random girl it was one that I had matched with on Hinge from before we were dating, we never met in person or even really flirted or sexted and just kind of fell off from talking. I was scrolling through my old instagram messages and saw her profile and came across one of her pictures and then started, I ended up stopping pretty soon after realizing what I was doing was very wrong and then ended up telling my girlfriend about it later in the day. I was already spiraling before that point and I believe the act was completely out of a need to self destruct and hurt myself without realizing I was hurting her. I realize that shouldn’t be an excuse for the action though. When I told her she was silent which isn’t unusual for her and she had said she wasn’t hurt and still trusted me but that I shouldn’t be surprised if she leaves if I were to do it again or actually cheat. I can’t fully understand why I did it considering how much I love her and everything I do to try and show her that and make her feel special but I know I can’t change what I did. I told her that i’m going to work on myself so that it doesn’t happen again and i’m actively doing so. She didn’t see it as cheating or breakup-worthy but I think it technically would be and I feel like the worst person in the world right now. Do you think it was cheating? Is it reasonable to think things can be repaired in this scenario?