There's only one way to get right to the heart of a matter like this one. You've got to let him go.
Did your stomach sink when you read those words?
Well, when I said "you've got to let him go" I didn't mean that you've gotta end the relationship. What I meant was that you should drop the unconscious or semi-conscious (or whatever) belief that your joy, happiness, self-worth or self-esteem..., etc. depends on whether or not he will ever abandon you for another -- or otherwise leave you or this world.
You're not going to drop that, however, unless you first see it clearly and find it to be false -- and this is only very partially an intellectual endeavor. It's mainly a feeling endeavor, and can only be approached moment by moment, ... by moment by moment... -- in other words, right now. That's all we've ever really got, right now, and our right nows are where we construct the habits, emotional or conceptual, that conspire to make how our lives go.
Every relationship should best begin with letting the other go, because when we cling and that clinging causes fear and resentment (etc.) it always messes up all that is good in these relationships. Clinging, grasping..., has exactly (precisely!) the opposite effect of insuring that the other will stick around in a loving, joyous, beautiful sort of way. And the irony is that letting go is probably the only and best way to keep the ones we love around.