Sleeping with the man who I’ve liked for years while he is in a relationship, will it ever work between me and him?

I have liked this boy for 3 years and always dreamt of being with him but he is my best friends brother so neither of us entertained it, he has been dating this one girl for a year but the last 6 months they have been constantly arguing and she refuses to have sex with him so my best friend who knows how much I like him told me to go for it and said that I’m the kind of girl she’d want her brother to be with, he messages me and we decide to give it go and ended up sleeping with each other, he said that he wants to break up with his girlfriend but is waiting for something to happen like a big argument because he doesn’t know how to break up with her as he hates confrontation n he tells me that he doesn’t want me to think that he is using me for sex and that he sees himself being with me. One of my friends told me thst “you lose them how you get them” and it’ll never work out and I also just feel guilty, I just want to know someone’s opinion on this situation