Only one letter separates the two words, but "wishful" is having hope for something, and wistful is having sadness or melancholy about something. "Wist" isn't even a word that's used anymore, but you can still be wistful.

People who appear wistful often show a longing for something or a look of serious reflection. One way to describe the adjective wistful is as the sad appearance of someone looking back and thinking "if only..." A thoughtful or pensive mood centered on something good in the past that is missed or something not so good in the present that could have been better "if only" something had gone differently — these things make for a wistful outlook.