You've asked, how do I leave a cheating partner?
First, do not beat yourself up because it's taking time. You may not be ready, yet, to end it. It can take some people a long time to be ready, but I think that once they are ready, they often move on very quickly, because they've already done all the thinking and processing. It's like they've been in the forest and suddenly emerge into the light.
Second, to help yourself get there, I'd suggest being brutally realistic about the relationship. You might benefit from keeping a note of his behaviours and your feelings - when he lies to you, any fights or cross words, when you feel anxious about where he is, etc. You need to see the relationship for what it's actually like, not what you had hoped for in your mind.
I wonder, too, whether you might benefit from building up your confidence by focusing on areas of your life that don't involve him. Friends, hobbies, exercise, perhaps planning for a new job? Could you take a holiday, to get a literal break from him? Someone suggested a pet, another great suggestion if you're able to take on the responsibilities.
Do you have a confidante, apart from your therapists? You need some people In Real Life to help keep your head and heart in reality, and to support you. Isolation is not your friend.
If your resolve always crumbles when you're around him, break up via text message.
Last few thoughts - you sound like you're driving the car with your eyes firmly planted in the rear-view mirror. You express a lot of regret about your past decisions. But you can't change those decisions, and staying in your current relationship because of what you sacrificed sounds like an example of the "sunk cost fallacy". What you can control is the choices you make today. So what will you do today, @LivingInLeggingsStill , to move yourself towards the life you want? You don't need to dump him! But think of one thing, however small. And do it.