I am so sorry to read that your marriage is ending...I hope your doing okay. It is a harsh slap in the face, when we ask questions, we may not want to know the answers too, such as your question, "what are we going to do to fix it?" and her response, "I can't". I have been where you are, but in a different way- i had a cheating ex...in the end, he was the one to leave me.- which devastated me to the core.
As you have stated, you are in control of your own destiny and you have a right to happiness and a loving relationship.

My mother told me, in response to my SOs past baggage, his kids and his nutty ex wife, "bloods thicker than water". I didnt really understand what that meant, and i began arguing with her- "what are you implying?", so your saying that, "if i meet someone else, their family will always come first and ill be second?" I thought that was BS and if thats the way it is, id rather be single, if im going to chronically be put second. (This hasnt happened yet, but it was a forewarning to me) 

Unfortunately, for you and your stbxw more, is he relationship comes first, family second. That doesnt mean that you feed kids last or if there is imminent danger you put them last. But the relationship should trump all (unless danger lol). It seems your wife is going to have to learn that lesson the third time, maybe even the fourth time, she may never even get it. 
If you cant see her changing, than its best for you to not waste any time and start living. Her kids will always be drags until she sets boundaries.
I know how step kids are, and most of the time its not fun and games.
Best of luck in your journey to recovering and healing, you will find someone who values you and puts you first :).

Your definitely on the right path- especially mentally- it can take a toll. I too wouldnt want to be with someone that wouldnt try either, it says a lot about how they feel about us...however, some people can change and maybe you two could work it out, but it would take much work...you know the best path to take for you. 
Best of Luck!

I'm sorry but not sorry- a few months ago I was where you are now, doing all I could to keep alive a relationship that was already dead. He too, would not listen and would do nothing to help us come together.

I did everything but beg, for his sake as much as mine, but there was no giving. I think he had made up his mind months if not years before, and when I realised that all the feelings he had ever had for me were gone, I walked.

Now I can hardly remember what he sounded like- he rarely taked to me anyway- and I am busy getting on with my new life. I am utterly grateful for my close friends and family who are there for me all the time. 

Life will bring you things you never thought of, so move on with confifdence in yourself and someone wiill come along you has the sense to put you first. I hope so.

I have no doubt that whatever you decide will be what's best for you. You've been struggling with this for a while and you have finally reached the "I'm Done" moment. I can imagine how sad this makes you feel but you know deep down that its the best thing you can do. You shouldn't be placed 2nd and the fact that she said "I can't fix it" says where she stands in this.

Wishing you much strength in moving forward.

I know this must be a kick in the *insert location*.

Just one thing I noticed from a previous post... this is your 3rd marriage. And the same scenario where you are treated as a 2nd class citizen. Please, before you date again please get to a therapist. You need to work out why you keep doing this to yourself.