Let it fly, don't hold back express yourself. It can't be any worse then most of what's been posted here by me.

I'm sorry you feel this way, it's a ruff place to be speaking from experience. I tend to feel like that when I'm mentally bored or fatigued, this often causes me to over-think negatively. Anything from the past that you found interesting, that you may want to learn or re-learn? or possibly a hobby?

Another possibility is diet, dehydration or a deficiency (vitiman, amino-acid/fatty acid, etc).

Hope you feel better soon.