Long distance/ history problems arising suddenly!

Hello, I am a college student and am dating a girl my age who I have known since around 13 years old. I started dating this girl a week before I found my father passed away while i was living with him. He was living alone before i moved in the week before he passed away. Now since then me and this girl became very close and she told me how she had never been so close to someone. she even told me how with her previous boyfriend of 3 years she cheated on him at least 5 separate times. This made me sort of worrisome but i let it go cause she was so good to me supporting me through my darkest hour and being there for me. then last summer at a party while were we all very drunk I walked in on her making out with another guy. we had a rather lengthy fight and then made up and i made it clear that cannot happen again. now since then she has been 100 percent faithful to me and has spoiled me so much. She drive 4 hours to see me at least once a month at my school and buys my concert tickets to my favorite bands. But tot he problem im having. I have been for the past 2 weeks not seen her once and have been getting terrible ideas about her cheating on me and the image of when i caught her kissing that guy pops into my head when i try to sleep and i break out into a cold sweat and my heart races. This has altered my mood and made me very confrontational with my girlfriend even tho she has didnt nothing wrong i will lash out randomly. I want to end this behavior but I don't know how its really hurting me emotionally and physically. I know she is committed to me and would never harm me she is great to me and i never would have been able to make it through half of (excuse my language) the shit that i have gone through with out her. I love her and want to treat her right by ending this terrible thoughts i am having thank you.