I think there are a few different threads in your OP, anothernamechanger, I just want to mention a couple of them. Its only my opinion, so feel free to disagree!
First of all the guilt 'cos you fucked up a couple of times. I think wishing you had done things differently to save yourself (or your kids) from abuse and grief is a real journey. Especially when you have gone along with things when deep down you don't agree with them now and you had reservations at the time. I think its worth identifying these. Was it naievity, ignorance, pride, wanting a relationship whatever, lack of confidence, self-delusion, and so on, there are sooooo many. And then I think you have to forgive yourself. And look at how much you have grown to do the right thing even if you didn't at first, or were unsure. The fact is you left and took your children from two bad environments and set out on your own to finally create one free from the kind of abuse and fear you describe.
Second, I think its a bit understandable to move from one abuser to another (or one jerk to another). I can only speak for myself here, but I really wanted the second one to prove that not all men were arseholes and I had it in me to meet a decent man like other women did. I doubt very much there will ever be a third, as I'm just too clued up now. And I bet you are.
CBT and Mindfulness. Some people love these tools. But they never did anything for me. I do not think CBT is helpful for everyone, especially when grief or abuse is concerned, or just seriously growing as a person. I found it very task-oriented and trying to change my behaviour to fit in with a desired outcome. But it is apparently highly popular, especially on the NHS, as its fairly short-term and therefore cheap. Mindfulness for me is also something I didn't find suitable or helpful, though many people do. It just didn't do it for me in any way and again I didn't really like the ethos around it (read Suzanne Moore for more clues on that I guess!).
Some psychotherapy counselling can be great, but I agree it can also be expensive and long-term. I've been through a few therapists overall, and its taken quite a few years. I would never want therapy again I don't think I can do quite alot of self-analysis, but I have to say it did manage to help me in a long struggle, plus it has helped alot with my personal self-awareness and growth generally. I struggled badly to get the money together, but sometimes if you are desperate you can find a way ....
It concerned me a bit that you are still scared about your ex, and don't have a lot to add on that one. But perhaps speak to Womens Aid for some more ideas or reassurance?
Truly, all the best to you.