You haven't given any details to suggest PTSD, which can only be diagnosed by a professional anyway and based on a lot of input about exactly what past traumas may have contributed. I have a friend with PTSD from time deployed as a Green Beret, and he also came back with chemical poisoning and other issues that caused major headaches, fears, etc. You have said nothing about your fiancé having any reason for PTSD.
A lot of people latch on to labels because they think it excuses behavior - "It's a disorder so he can't do anything about it."
It also sounds a lot like he has a major anger problem. You've been posting for several years about his anger and his reactions to things like what children say and what clothing women wear.
You also have referred to him as your "soon-to-be-ex" but now he is your fiancé - unless this is a new man. You also have an ex-husband who has been a problem.
I think the problem is the choices you make in men, and the way you behave in those relationships. You have children with these men, and then you feel stuck with them. You are so tentative in your actions and words because you are afraid of what the other one will do. You said it well here too - you are now anxious about talking to him about anything. You think discussing a problem is attacking him, or that he will see it that way. That is NOT the basis of a healthy relationship.
Whether you have anxiety or co-dependency, or both, or neither, is not the point. You need to get help right now. Maybe just for you (and your children) but maybe for the relationship. You cannot control what this man will do. You can only control what YOU will do. To start with, accept that you cannot solve this yourself, and you cannot solve it on Mamapedia. Look over your prior posts - I took a quick look just at your titles going back to 2012, and I remembered quite a few of those questions and the many answers your got from moms on this site. They all dealt with some self-analysis and getting counseling.
Now you are saying you have "suggested some counseling for both" but then you question that and say YOU are the one who "needs to move on" in thought processes and accept his efforts. You have not been happy with this man for years, but you think you have to accept that what he does is as good as it's going to get? He's angry, any little thing sets him off, and you have children in the house (one not his), and you think you just sit there and live with this? Sounds like waiting for a bomb to explode. Not wise. If you won't get out (and it sounds like you won't), then get help. Don't wait for him to agree to go. Go yourself. Call your physician for a referral to a counselor who accepts your insurance. Do it today.