Be careful of self-fulfilling prophecies...

The main reason your worst nightmare will become reality, is because you're hung up on your ex.

You said it yourself, you keep waiting for him to change his mind and come back to you. So no wonder your attempts at dating were feeble at best, you're simply not available to other men, and will remain alone until you are.

What to do?

Get in touch with reality (I mean that nicely, however blunt it may come across) accept that your relationship is over, remind yourself that there are many good reasons its over, and make a conscious decision to move forward.

Being "friends" with him is doing you no favours. He occasionally whispers sweet nothings in your ear, knowing it keeps you hooked, just in case he's tempted to stray from his marital bed, that way you're ready, and waiting. (Hoping it means he's leaving his wife).

You, you're not much better at this "friendship", you're pretending to be his friend, waiting for any signs that his relationship is on the rocks and that he's changed his mind.

There's a reason people take a "no contact" break after a relationship ends and they both wish to remain friends.

Its for an adjustment period, to grieve the romance, and adjust to seeing that person as a platonic friend who has a SO.

If the two of you really want to be friends, you need to break up properly first.

(Personally, I seriously doubt that's possible under these circumstances, a clean break is in your best interest)

I know I'm very direct and I mostly suck at sugarcoating, but I'm posting out of concern. You've suffered enough, its time to put an end to this, but only YOU can end your suffering.