Has anyone ever tried to write an "emotion" journal?

I've just bought a journal where to write how I feel and all my thoughts. I don't know if I'll ever let someone read it. Probably the therapist. But has anyone done it already? Has it helped you? Because one of my worst enemies is my memory, or lack thereof... You think it should be best to focus on just the fleeting thoughts or collect also flashback that I deem important for my current situation? Maybe divide them, so as to have the rational and the emotional side.