Doubting,confusion and anxiety all are symptoms of ocd , but i cannot diagnose you here, you need to consult a psychiatrist for proper diagnosis.
Regarding your course in medicine, i would advise you not to leave your course,the problem is not with the course,the problem is in your mind, you need professional help.IF you shift to another course then what is the surety that these thoughts won't come back.
And the best way to tackle these thoughts is to just ignore them , you cannot to anything more because these thoughts are irrational and you cannot argue with them because again your mind will find a loophole.
But if ignoring is not helping much then you need to consult a psychiatrist
I also can never decide what to do with each day because I think there is one right way to spend it and maximise things but I don't know what that is so I get really anxious and don't know what to do. Then I do nothing but feel so guilty because I've done nothing. Then I question if what I am thinking is real or if I'm just pretending or making this up to get out of doing things and having an excuse. I don't know if this makes any sense.