OP - I left the "B" 9 years ago....and honestly he is not a BSTRD, just entitled and a poor communicator. I of course have my faults too.
I don't regret being single, I truly enjoy it, and I'm soooo bloody independent now, I can't really imagine ever wanting to live with anyone ever again, or even be in anything more than a casual relationship. In exchange I work all the time, cannot be a SAHP (even if I wanted to), have many jealous women be snipey at me, or try to convince themselves that I might be jealous of their (unattractive) husbands !! I'm in Ireland and currently working for a very conservative right wing company......so I'm probably experience some unusual backlash that the average separated woman may not be subjected to. In general, I notice when you are a free, independent woman, lots of people in insecure relationships want to degrade you a little, as you are holding up a mirror to their lives and showing it is possible to separate and they feel threatened by that. You have to be mentally very strong to deal with the burden of full financial independence and also societal disapproval.
Please exercise a little bit of caution and realize the feedback you get on Mumsnet is something of an echo chamber. It is all women of a certain age/space in life, who are all dealing with similar stresses and are not viewing this from a completely objective perspective. There can be an awful lot of projection as well. We are hearing one side of the story with snippets being shared in frustration and as a method of releasing. It's very easy to type LTB, and escalate everything out of proportion.......the reality of life is much different.
Before you LTB, or secretly plan to LTB, consider the alternatives. Better communication between the two of you. Counselling for yourself. Increasing your hobbies and inter-actions with others outside the home (would this help)?
Working towards more financial independence.
Tell your life partner how unhappy his communication style makes you feel, tell him you need to feel valued and he must be more empathetic to your needs. Tell him if you are in pain, he needs to realize that is a problem for the entire family and he needs to arrange the dentist, that is the agreement between two parents (one working in the home and one outside the home). You are holding up your end of the bargain by parenting full time, he needs to reassess his commitments to providing for you (that means providing and facilitating healthcare).
Setting up businesses when you have been out of the workforce for a long time is a very lofty aspiration. As you know, you will most likely need to work in a salaried position first. Do you know what the going rate for a position that you are genuinely likely to get is? Check this out first. Then you will be working full time and also be a parent, assess how much time you can devote to building a business after all of life's other commitments. Yes, it's possible, but really be prepared to view your future plans with a balanced perspective.
Take care of yourself. I hope my post will be received as it was intentioned, simply to take some of the wind out of the only option of LTB....consider the other options.