I remember having sleep issues since I've been a young teenager. I generally over-sleep and when I'm awake I almost always feel tired. I wasn't referring to occasionally getting restful sleep but an overarching pattern of "bad sleep" throughout my life.

It's known that depression can cause a person to oversleep and that mania can reduce the amount of sleep that one gets. At one point I'd theorized that part of bipolar disorder was a neurological sleep disorder since it seemed like commonplace that members here are often taking something to aide in sleep. Lack of overall restful sleep can also mimic ADD/ADHD. If sleep damages cognition, then it would make sense that even when you have a restful night of sleep the residue from the bad nights would still effect you despite how well rested you feel.

They also report that psychotic breaks can deteriorate cognition, too.

I hope you're able to find a therapist, soon, who can give you some insight on this problem.