No judging please

Me and this guy kinda have a thing going on, but we are just friends with benefits.We both are in relationships with really great people but we aint happy in the relationships.I gonna come clean to my bf.Later this week when I see him. But I dont know how to tell him I have been seeing another guy, behind his back. I know what I have done is wrong,that's why I'm telling him,I can't keep letting him think that I'm happy in this relationship.