Your WORST fear? That he will meet someone else and (God forbid) be happy?
What he does--whether he is with someone or is happy or not--has absolutely NOTHING to do with YOUR ability to be happy. His being happy doesn't somehow deplete the amount of happiness available to YOU. That's math that doesn't add up.
There is plenty of "happy" to go around. I hope you will start going for YOUR happiness.
One thing that is stressed in AA (because it's literally a matter of life and death for us) is equally important to people recovering from relationships with alcoholics. Resentments are bad news. Resentments are like drinking poison hoping that the other person will die. When you can let go of resentment about whether he's happy or not, or what he may be doing, you will lighten your own load considerably. Seems to me that life gives us enough baggage to lug around without carrying someone else's.