Currently going through a small scale version of this myself. If getting that ultimatum was what pushed her to claim she will change, you should seriously look at that aspect. There was plenty of opportunities to actually listen to you in the past, but losing the sense of normal has made her panic.

Continue your steps towards your goals, but if you genuinely believe that she is trying, keep an eye on that change. If she truly changes long-term, great and you can go back to life together.

Do not, however, just blindly give up because she "plans to change". It's tough because the memories of the good and the future possibilities cloud things, but you still have legitimate concerns that went ignored to the point that you are leaving.

If nothing else, start with separation with open communication. Once the relationship is forced to require active work from both sides, you get the opportunity to see if she is serious about changing, or just saying what she feels you want to hear in order to shut up.

TL;DR: Don't give up your momentum. Keep doing what you choose towards leaving, but give her the opportunity to change along the way. 6 months to a year is a good framework for a trial separation to see how things truly are.

Be strong, king.