I'm glad you have recognized the "Nice Guy" routine. Seems like you have identified with yourself. Here is the kicker though...
You have to FIX YOUR behavior. You have to show that you are changing. You really have to understand, that your W will not trust you until she see's you changing once and for all. THIS takes SERIOUS amounts of time. Time that you may or may not have. DO NOT BOTHER TELLING HER. SHOW HER.
You have to change for you and not so much worry about dropping an Ultimatum RIGHT NOW. Seriously, with EA/PA can it really get worse at this moment? Don't focus too much on enforcing this information just yet...take some time and soak it in. In the mean time just be distant and BETTER yourself. Don't worry too much about enforcing a boundary when you have SO MUCH to do for yourself.
I truly believe in showing your best self and getting acquainted with YOU. Once you do this, Boundary setting and what's BEST for you will make everything much smoother. I think you of all people can identify extremely bad behavior, but until you handle them correctly just play it day by day until you grasp EVERYTHING. Then think about setting an Ultimatum.
Read DR...if you already have...do it again. Just my opinion. It's all about doing EVERYTHING differently.
