I have been in this situation before and it's incredibly painful.  You're not happy, you're not unhappy.  You're not together, but you're together (and sometimes in bed).  He's kind of seeing someone else and kind of not.  Is this any way to have a relationship?

I know this is really hard to do, but you need to end this.

It's the only way to get over it.

  From what I can tell, you can't seem to part because you can't let go, not because the relationship is so wonderful.

Set up a final good-bye meeting if you need to have closure.  Tell him how wonderful it's been and how hard it's been (without blaming anyone).

But HE is to blame..

Cry if you need to.  Then, walk out the door.  Do not send tapes or letters.  Don't call.  You are going to "miss him constantly" at first, but it will get less and less painful as time goes by.

Set up several fun things to do with friends and/or family for the week following the good-bye date.  They will help you get through this.