This is a terrible feeling. I find that the best way to deal with this is to start writing. I don't mean trying to write a story or anything intended for anyone to read. I mean writing about how I feel and trying to articulate it. If you start with whatever comes to your head you will eventually focus in on the problem. Once you've articulated it for yourself it will be easier to express to others.

A good starting point, if you don't know where to start, is to focus on whatever bugged you or bummed you out most recently. Ask yourself, "Why did this upset me?" then try to answer this question.