The same kind of thing happens to me, but not quite so suddenly. It takes a few days for me to come down like that and it takes even longer for me to get back up again. I also have problems with all types of relationships. Whenever I do come down, after the initial shock and long rumination of how I got there, I try to look forward to the time when I will be up again. Because if all went right, what would we have to look forward to? I have no idea how to make it last, but I imagine that when you are where you're supposed to be, that happiness will be easier to hang on to.