Brother,
As @AmirAlmuminin Lover said, we should be rational. We believe that jinn and other ghaibs are present. Sihr and black magic are also present. Waswas (the feeling that we can't focus all the time, always worrying sth, etc) is also Shaytan's deed for us.
Black magic, as far as I know, is sth done by person by the help of jinn to disturb others. We should differentiate between waswas and black magic.
If we blame black magic for ourselves being not able to focus, we are blaming someone other.
If it is waswas, we must increase our zikr shield. Especially in the morn