I’m so very glad you wrote. You are right to be concerned. It was wise to reach to us for some help.
Let’s start with how tired you are. Your difficulty with concentration and focus may be an outcome of your fatigue. At 15, this level of exhaustion isn’t normal. Please start your journey to health by seeing your medical doctor. It may be that there is something going on like an iron deficiency or vitamin deficiency (or something else) that can explain it. When someone is this tired, it’s very hard to cope. Fatigue is one of the ways that our body tells us that something is wrong and needs attention. Trust it.
As for the other symptoms: I doubt you are suffering from schizophrenia. For one thing, you are young to have that diagnosis. The talking in your head and counting things may be a creative way you have found to get yourself through the day when you are so tired. It may also be how you are containing anxiety – which at your age is a more likely diagnosis that a psychosis.
What concerns me most is your report of cutting and your apparent low self-esteem. I congratulate you on doing what it took to stop self-harm. The next step is to do something about the feelings that made you do it.
For that reason, I do encourage you to also seek out help from a mental health counselor. You need and deserve to learn what has prompted you to feel so negative about yourself. You need and deserve guidance and support while you work on mastering better ways to cope when you are stressed.
I wish you well.