I go through the same thing very often. Do you suffer from borderline personality disorder? I only ask this because black and white thinking is a common characteristic of that. When things are going good my thinking is great most of the time, but when things start going downhill for me my thinking is really bad.

All I can suggest right now is to challenge your negative thinking...it's really hard to do and believe but it seems to work when you do it on a regular basis.