Someone please tell me.. is the gr greener on the other side?

been having crazy ual thoughts about someone other than my boyfriend - this has been killing me for a while - longgg while. been with my bf for a loong time 7 years and there is this one guy who comes in and out of my mind for the past 2 years on and off i just want to sleep with him so bad! and sometimes it drives me crazy that i can't get him outta my head - i feel horrible - anyone been in a situation like this? is it better to act on it? one night stand? or just forget about it?? & if forget about it how TF do u do it?? its bad