He really wasn’t worth all of that anguish was he? What a good job you did have an abortion though, because really, to be free of any ties to this guy has to be a good thing.
It’s not uncommon for people to feel a spark with an old school friend - you meet up years later and see how each other has matured, has become more confident etc., and wham bam, you can’t help yourselves.
But this guy is taking it too far - you’re absolutely right to deny him the best of both worlds. Just try to let go of the desire for revenge. It’s natural to want to hurt him and to want to help this other woman see what she’s getting, but my advice is to stay out of it, keep a clean conscience and just get on with your own life.
He isn’t adding anything positive to your life at all, he’s just negative energy, draining your own confidence. You will be jealous, angry and hurting - anyone would be. But focus on what you stand to gain by getting him out of your life.
It will be you who’s choosing to part for good, so you should take confidence from that. He’s not dumping you. He’s just weak and wanted everything he could get.
He probably doesn’t want to get married to this girl but that’s not your problem. He’s going through with it for some reason, so best leave him to make his own mistakes.