What was I thinking...

Hey guys, so I'm feeling down about myself! I am happely in a well over a year relationship and this past weekend I meet my best friends brother for the first time... at first he seemed cool and then we started flirting. I know I should have stopped it but I guess I just liked the attention. Things started speeding up and we got really close emotionally and physically. I ended up sleeping with him... (and not just once;) we both have other partners but we both are falling for one another... I don't know what I should do, the weekend we spent together was the happiest I've been in awhile, and he's everything I'm in need of when it comes to companionship... what do I do 😢