Keeping a journal is one way to monitor your mental state regarding poker. The drawback is that the written words can't quite convey exactly how you are thinking/feeling during a session. Here's an exercise that might help anytime you've been playing poorly, or just struggling with poker. I tried this yesterday after a couple of bad sessions, and it seemed to help.

All you need is the free version of http://audacity.sourceforge.net/, and a $10 microphone.

Exercise:
Simply talk freely about anything that pops into your mind during a session. Go over your thought process, how you feel, plan of attack against different opponents etc. Remember, nobody else will hear the recording, and it's not intended to teach others. It's simply a verbalized version of your thinking process. Think of it as a casual conversion with yourself about poker. Listen to the recording later, and you might make some interesting observations. How does your tone sound like? Did you gloss over hands lazily? Did your emotions change? Did you berate yourself?

I found the exercise liberating. We will have a relationship with ourselves forever, so why not verbalize it once in a while? It could clear up issues.