Are you talking about difficulty concentrating when your NOT masturbating or while?
Im 18 and seem to have the same problem, i used to be able to concentrate and work hard for long periods of time, recently (as I begun to masturbate and have sex more again recently) I am finding it hard to revise and work, I think you might be onto something there
In my experience, the fatigue does. Addicted to sex, well you can be addicted to any behaviour, though ejaculation/orgasm causes potent extensive alterations to your body (no specifics), obviously i'm suggesting it might affect your cognition.
If you masturbate to much, you can become addicted to sex. So, neurotransmitter dopamine increases in synaptic cleft. It affects mood. And when dopamine is not here in quantity you have during masturbation you can have difficulty concentrating, because your body learned to function w/so much dopamine...