A [M/20] feeling like a fucking piece of shit for hooking up with a random girl right before making things official with my girlfriend [F/19]

Please read the whole thing so you understand completely. If you only read the tl;dr, mention in your comment. Thank you.

3 years ago I became friends with a girl (Girl A) I met at a summer program for high school students. She went back to her hometown, I went back to mine. Over those 3 years we texted, FaceTimed, and told each other everything, from our issues with depression, eating disorders, really anything... I feel like I can't really hammer it home completely but we became best friends without ever really seeing each other at all. When college came around, I ended up going to school in the same area as her, as she had dropped out due to some issues. I joined a fraternity at my college and began inviting her to events. She would sleep over, first on the couch, then in my bed, but we would never touch each other. I had feelings for her but after 4 or so months of just hanging out and being deathly afraid of making a move and ruining a friendship, I gave up. Suddenly, she asks when she can come around again and she does, spending the night in my bed again and we finally kiss. We end up being hookup buddies that are also best friends for a month or so, and I cut everything off with every other girl I'm talking to. This month, February, things get pretty serious, as I meet her mother the Thursday before Valentine's Day, she sleeps over, and we profess our love for each other. That Friday night she goes home and I go out to a fraternity party. I drink too much and encounter a girl (Girl B) I knew from my freshman year that has just broken up with her boyfriend. We began grinding on each other and hooking up. I took Girl B home, and we got semi-naked on my couch and made out for a long time. We did not have sex (confirmed by Girl B), although I believe there was some genital touching in there. I woke up the next morning with little recollection and asked Girl B what happened and she confirmed the above.

I felt awful and realized that Girl A is the only girl I want to be with at this point in my life. I've been in several serious relationship and have many, many insecurities and trust issues, and Girl A accommodates all of them and loves me pretty unconditionally. The next day - the day before Valentine's Day, I told Girl A that she was all I ever wanted, professed my love for her again, and made her my girlfriend on the spot. On Valentine's Day I took her to a fabulous rooftop restaurant and we took a walk along the pier, and it was perfect. I felt better about it. 2 weeks have gone by since then and things have been great.

This morning I awoke from a nightmare where Girl A told me that she had hooked up with someone even earlier than I had, and it crushed my soul. I realized it was only a dream and was relieved... but then realized I had done that, but more recently, and in reality, to her. It's been eating at me all day. I have talked to 2 of my best friends and 1 says I must tell her, the other says "technically you weren't dating, you get 1 free pass, let it go and treat her like a princess forever." The other friend says she'll understand if she is as great as I say.

I am her first real serious boyfriend and know what it's like to be cheated on at first. It changes your life trajectory forever and makes you have serious trust issues. That's what I have and it's taken forever to find a girl that will accommodate those insecurities, and I feel like if I disclose that I cheated to her, she too will have those insecurities and not let me care for her and protect her, because I hurt her.

The fact that I did it is eating away at me. I can't imagine going on for months (hopefully years - I really love this girl) with this on my chest.

WHAT DO I DO?

tl:dr I began an exclusive hookup with my best friend, fell in love with her, then hooked up with some random girl after, then realized I only loved and wanted to be with my best friend, made her my girlfriend, and feel like I cheated. PLEASE READ THE WHOLE THING SO YOU UNDERSTAND COMPLETELY.