I'm sorry you are feeling down at the moment.
What you described is quite a common symptom of depression. In the
Diagnostic and Statistical Manual of Mental Disorders (DSM-5), the manual used to diagnose all mental illnesses in the USA, "diminished ability to concentrate" is listed as a symptom of depression.
Could you be depressed? Have you ever sought treatment for depression? Maybe you should visit your doctor and discuss your symptoms with him/her.
I hope you can feel better soon. Sending hugs
.