What Causes Itch On Scalp ?

Hello, I am a 20 year old girl and I have had a quarter size spot of itch and dry white flakes that I tend to pick off when I get really stressed. This all started my freshman year of highschool when I was really stressed from the transition. It has since grown to almost the whole back of my head . It sometimes itches really bad and thats why I pick the scabs off but sometimes it just because I am nervous. I went to a dermatologist and they prescribed topical solutions and a pill for only 20 days due to the damage it causes to the liver . When i was on the prescribed pill it seemed to reduce the scab almost completely but once off of it the scab problem came back. It usually irritates me the most after I shower. I usually pick the scab off right away after showering no matter how many times a day I shower I will always pick the scab off. Very rarely do I get in the next shower without picking the scab off. Its very embarrassing and Its not dandruff or porisis I feel like but I just don t know if it is only stress induced or if there really is a problem with the skin and if it really is treatable. I just wish I could get rid of it. If i take a shower before picking the scab it will come back harder and be harder to pick off so I think if i could not pick it for like a week it might heal itself I just never give it the chance because it itches so bad or I get so stressed. If i pick the scab off more than once or twice in the same day it will bleed a little. please help I have tried every treatment and shampoo advertised and nothing has worked or helped not even the dermatologist, they even tested a sample to see if it was a fungus and it came back negative so i have no clue how to get this off my head and stop the embarrassment.