[MF] Hooking up with an old friend (cheating)

This is my first post so bare with me.

So to begin, I’m 22 and have been with the same girl for 4 years. It’s a decent relationship and I’ve never cheated before. So this is a first for me
I got in contact with an older friend recently. We decided to meet up for lunch the other day. We were just talking a bit and I helped her with studying for a test she had in one of her summer courses. We were talking a bit and it started to get a little flirty. I didn’t think too much of it.
We leave from lunch and I walk her back to her car which is in the far end of the parking lot. We’re talking and I’m checking her car out (it’s different than the one she had before). I open the back door and she brushes really close by me and my face to move a few things around. Her face was really close to mine. While she was bent over moving stuff around, I couldn’t help but stare at her perfect ass. I don’t know why but I reached out and grabbed it. It made her jump and gasp. She turned and looked at me, we were close again. I just went for it and kissed her. She kissed back and we were immediately making out.