Our daughter is 17 and she knows mom for what she is, she's even told me I'm a saint for staying with mom. Our son is 19 but special needs with some intellectual disabilities.
So there are no custody or parenting issues that she could try to use against you like mine did when my son was 3 years old. There were so many allegations to children's services, hospitals, etc that I lost count. They all were deemed "unsubstantiated". Sadly, a more actionable word would have been "unfounded", but it seems professionals prefer passive conclusions.
Your children are old enough to know the reality, that you're not abusive nor the one aggressively threatening lies and repercussions. However, they've lived with this dysfunction and discord for their entire childhood. How have they been impacted? Are they seeing counselors? (Highly recommended.) Would they seek to live with you if the marriage does end? Would they defend the reality or would they wilt and side with aggressive mother? Children, even grown, prefer to avoid taking sides.
I know she can be extremely vindictive, she will lie. She's taken revenge on others who have wronged her (posted lies on social media). She's said things like she'll ruin me many times. There are lies she even believes. We've had full conversations/arguments over her saying I said something I absolutely did not say (but she 'felt' it so to her it was true) - she even has journaled things that didn't happen or things I said that I didn't say (some conversations were recorded so I could go back and made sure I'm not crazy)
Which is why I recorded, back then it was with voice recorders. Technically, to play it safe, I say "I recorded myself to document I wasn't the one being aggressive or abusive. If others were recorded being aggressive or worse, well..."
My ex was similar. I recall one time when we were volunteers, about 6-7 years into our marriage when she had a serious tiff with a co-worker. The manager counseled us but she denied it. Afterward she was angry with me that I kept quiet and didn't support her Denial. I said, "But that's precisely what you said to me a couple nights ago!" She denied again, saying she could talk confidentially to her husband how she felt but she would never actually do it.
So weird, she too claimed to be a victim but I never ever saw a more
aggressive victim.
I am still trying but I'm not sure she will hold on unless I drop therapy, stop any work on my codependency and go back to being 100% her caretaker.
Sorry, you can't undo what you've learned.
With the children grown or nearly so, you have more freedom to stick to your newfound boundaries.
What other options do you see besides caving? Many here realized that adding distance to the relationship (separation/divorce) reduced the conflict. You both would still be parents, that won't go away, but do you think your life would be more peaceful with distance apart?