Zendog... you said it all for me. I truly believe they know what they are doing. I also agree that they think and believe ... "it is for the best"! I know for a fact, that they are right... accept as you said... for everybody but them. We all learn, grow and bloom. They wallow in misery until they figure out...THEY are responsible for their own happiness.
We figure that out quickly, right at the beginning of this. They... poor, lost darlings, figure out way towards the end, if at all, when it is too late. Mind you, if they are able to FORGIVE themselves, they are able to find someone new, not some sordid OW, who will settle for "whatever that is"... but a real, loving person. They can have a brand new, good life, but I believe they will always feel some regret, for the life they let go.
Hugs Stayed