I have a boyfriend who i care about, but i think i have recently established a crush on someone else?

I have a boyfriend of a long time, and recently i have started hanging out with a group of old friends from like 6 years back, one being an old crush i had, he does have a girlfriend and even though i have a boyfriend, who i do care about, this other boy has been a constant on my mind every day, the old feelings have returned, i've dreamed about him everynight, i dont know what i am feeling, or how to control it? I'm not sure what i should do, or not do? i dont know that the old crush has any feelings for me or not, hes only said and acted a few times, but im not sure if there was any seriousness to it? What should I do? Is it possible/wise to leave a long serious relationship, just because he has reappeared in my life with many uncertainties? Could it be lust for something new, because of how long i have been in my other relationship? i am so confused?