I've been there and I know how that sucks. In those cases I figured it was best to take my loss and get it done and over with as soon as possible (I'm sure that's what you're doing).

In my experience there's one thing you need to watch out for, and that's being a nice guy. You might start feeling sorry for your client because stuff is taking longer than planned, which makes it harder to say no to just minor changes to the scope. They will only take you 5 minutes each, and it keeps the client of your back. Those are the things that really get to you when it comes to staying motivated I think.