I'm cought in the middle of a problem between two women!?

Ok so I have a gf and I've been talking to this other girl ive known for a year and she has a bf but I told her I liked her tonight cuz I was with her amd we were all flirty and stuff and she said she likes me and I told her tht I'm not the kinda guy that will cheat on ur gf it's just wrong! So I told her in the future when we r both single tht we can date she agreed but my bestfriend had hooked up with her I guess Thts what he calls it (they just made out and stuff so Idt it's hooking up) but idk how to tell him I like her I didnt mean to fall for her it just happened and I feel horrible because I have a gf but at the same time I feel great cause I told her I liked her... And she likes me too but we both r in a relationship we promised we'ed keep the attraction to eachother between us cause we don't need ppl knowing but still I was looking at her profile pic on fb and she's so beautiful and pretty I wanna date her now!!!! It's just we're both in a relationship and my bestfriend had a thing with her... Idk what the f�ck to do!!