She cheated, kind off? 19M 20F

So im 19M and she is 20F , we have been together for almost 3 years, but last winter, half a year back, we broke up, we were fighting etc. (i dont remember everything i was on benzo medication) and after like 2-3 weeks we got together again. But in the meantime, she managed to sleep with a guy i knew, we werent friends but i knew him, i still dont know why did i go back to her, she promised me everything, and she was nice to me, she still is, but i cant stop thinking about that guy, cant sleep in her bed knowin what was happening there, and she keeps saying it doesnt matter because we werent together. Yes she says that it was a bad decision but i just cant take it. Now i feel like i cant get out of that relationship because its been so long i am scared to be alone.Also our sexual life got worse, i dont enjoy it that much. Please suggest something.