How do you know when it's time to leave?

I've found comfort in these forums but am also realizing the journey I'm on with my husband may be a hopeless cause. Do things change for couples? I went to his doctor with him a month ago. After crying to her that he didn't want to lose his marriage (after 3 affairs, $100k+ racked up in debt, huge tax problems, etc I am so ready to give up) she told him about this site, the books he could read - or even better, listen to on audio - as well other ideas. In the 4 weeks since that appointment he's done nothing. He asked for my old iphone to listen to music, goes online to watch soccer but will do nothing to help himself or our marriage. Is there anyone thinking of leaving or who has left and is doing ok? I'm actually pretty shaky now.