It sounds like your ex is one of those relationships that "keeps on giving." In your case though (like with most exes), all he's giving you is a headache!
There are two schools of thought about "can't hit" dreams. The first says that, because the body actually is paralyzed during Rapid Eye Movement (REM) sleep (so we don't act out our dreams), the dreams really reflect physical feelings of paralysis. When you try to throw a punch and can't hit, or if you try to run from an attacker but your legs won't move, what you are feeling is the natural paralysis of your body during REM sleep.
But how then, do we explain other dreams -- of running and jumping, flying and soaring, dancing and making love -- in which our bodies behave naturally, or in some cases, supernaturally? It's a fair question -- so fair, in fact, that I think we can be suspicious of a simple, "physical" explanation for your dream.
The only time you have difficulty moving is when you prepare to throw a punch at your ex. Is it possible your own good will is "holding you back"? At the beginning of your dream (and, we gather, in real life from time to time) you agree to see your ex again. Almost immediately though, you sense your mistake. He begins to degrade you, and you have fallen into the same old trap.
What's the message of this dream? It's time for the knock-out punch, but it won't be delivered with your fist. Rather, it will occur when you decide in your heart to let your ex take care of himself. You're not responsible for him anymore.