I think they make it all a mess. As far as I know, you've lost all your "temple blessings", which would include any sealings. But if you rejoin the church and go through their restoration of blessings non-sense, I'm sure that would restore your sealing.
You no longer have "temple blessings" which (as far as I can tell) consist of pretending you're name is what they tell you it is, getting a bit of salad dressing splashed on you while being mumbled over, sitting through the worlds most boring movie, while wearing a weird outfit and exchanging handshakes with some old person volunteer worker. You get to rearrange the weird outfit when Stan tells you to, and not before! Then you go and hold hands through a hole in the curtain with some old guy who mumbles some other crazy stuff about your bone marrow and belly button. Finally, You then get to walk through a door and sit in a room with an incredibly expensive light fixture. Next you get shown out of there and it's back into the real world you go.
And there you have it. Temple blessings. Oh yeah, you also get a card that may or may not allow you to have certain unpaid jobs at church. You will be allowed to go see extremely boring weddings, and will be constantly guilted for not going to see the free movie that you paid 10% of your income to go see.
Live your life. If you're happy and effective as a single, so be it. If you find a man that you'll love and be permanently happy with, marry him in a ceremony that is meaningful to the both of you.
Put this stuff behind you as pro-actively as you can. IMHO, that means ignoring as much as you can, which varies in degree of complexity for everybody. Pay more attention to eliminating "Mormon-think" and lingering attitudes and loyalties. I am not blase, as you probably have to deal with Mormons, and they will shove it back in your face.
"Living well is the best revenge," they say, and that means moving on with your non-LDS life. Worrying about silly myths like whether you are "sealed" to somebody who is not a part of your life is counter-productive.
From the church's perspective, nothing can break a sealing except a temple divorce. Resigning from the church, a legal divorce, etc none of that matters to the church. They believe you're still sealed to your spouse.
From perspective of everyone outside the church, that "sealing" means nothing at all. It holds no legal authority, pretty much anyone outside the church would consider the idea that you are still tied to your ex in such a way to be silly. What Godly being would do such a thing to a person. It's laughable.
It's also somewhat telling to me that a temple divorce doesn't involve a prayer, there's no ceremony, there's no pleading with the lord to break the bonds that once were made or anything like that. The temple president just signs a paper and a database record eventually gets updated.
I do understand how disappointing it can be to find out that there's still this link. It's just another nail in the coffin for the "truthfulness" of the church that they would do such a thing. If your ex is "holding this over you", I would make it very clear to him that if he really believes that, you can make his eternity a very, very long eternity indeed and that he might want to look at changing that himself.
My letter specifically says I am no longer a member of the church of cheeseandrice of battered gay saints. If I am not a member and can no longer go to the clubhouse and play Satan Says, I'd say my pretend sealing is canceled.
I know several people who resigned and their spouse remarried. And they did not have to get a temple divorce for their spouse (one of them a woman) to be sealed to someone else. So I'd say that means you are no longer sealed.