How do you leave your best friend that you love ??

Everytime I try to leave this guy, (3xs) he ends up sucking me back in. Acting like he needs me and loves me so much. It's hard to let each other go because we are close friends and actually planned a future together. We see each other all of the time and it's really emotional for the both of us to move on from one another. He gets jealous and territorial whenever I talk to someone else and vice versa. It's not in a bad controlling way. Just in a cute noticeable way any boyfriend will do. I know him so I pick up on a lot that he does. We've always been friends, but I remember the day we noticed each other as more. We never acted on our feelings until one drunken night, we were all out with friends and we had the liquid courage to express our bottled up feelings for one another. We've been through so much and have bonded through our experiences and I've helped him through things. He's really like a best friend. We love each other dearly, But I have to leave him. 
I know my feelings run deeper for him. I've dropped everyone because I believed in us (and because they were jerks.) I'm only leaving because I know I have to. Not because I want to or because he treats me bad. But after three times, I've run out of things to say. He says he loves me and wants to be with me and he can't wait for us to start a life together, but he's "waiting" to leave a girlfriend that he claims he doesn't want to be with, doesn't love, and can't see a future with, yet he's still with her. In the very beginning of us talking, I told him he needs to choose because I don't share no man. He said he wanted to be with me and to just give him a little time because of his kids. I honored that little time and was patient. I figured if you love someone, you're willing to go through the trenches with that person. So I figured it was like a test to see if I could handle sticking it out.. But here it is over a year later and he hasn't made anything happen yet. I don't want to hurt him, but I don't want to be in this situation because it's aggravating me that I even allowed myself to be here.. I'm becoming moody and depressed and it's not healthy. If he didn't wanna be with her and wanted to be with me, the solution is simple to me, kids or no kids. He makes it seem like he's waiting for some gigantic argument to happen for them to break up. Whatever. I know I need to leave, I just don't know how to do it when it's so emotional for me and I know how much I love him and want to be with him.