It was a mixed process for me. For the first 10 months or so, I was driving R more than STBX was, and he made a lot of the usual bad choices in R that a WS does (false R, trickle truth, lies). It was when I was ready to give up that he turned a new leaf, and for a few months, it seemed like there would be real change, then it all fizzled out. He didn't cheat again, but he became more self-centered and lost his focus on R.
I stayed in the marriage a while longer, watching how he behaved, but I knew partway into year 2 that it wasn't likely to work out. It became increasingly clear as time went on that he couldn't change in the ways that I needed - he was unable to let go of things he wanted, even if those things bothered me or hurt me. That was the ultimate deciding factor.
How has it turned out? Well, the vast majority of his behavior since we decided to divorce has reinforced that I made the right decision. Even the little glimmers of change seem to have regressed back to the impulsive and self-centered ways of the past.
We're still going through the divorce process (the hardest part is done, but a lot of stuff needs to be split up still), so I feel a mixture of relief and sadness. Relief that his problems will no longer be mine, and sad that he couldn't pull it together to save our marriage. My heart may hurt, but I know that I'm making the right decision for my health and my future.
If you're sure that your WS's actions are showing you that she's not a good candidate for R, what is keeping you in the marriage?