Psoriasis.

This shampoo helped me (mostly) get rid of dandruff and itching: https://farmakom.ua/en/products/shampun-digtyarnij/
So a birch tar shampoo.

However, after 3 years I finally decided to go to a dermatologist. It was getting bigger and bigger, and now my hair wouldn't cover it up. She prescribed me some corticosteroids (betamethasone 0.5mg/g with salicylic acid 20mg/g) for 10 days (twice a day) and that did it. But I consider corticosteroids a last resort. It causes skin thinning and with long term use it may cause topical steroid withdrawal when you stop using it. Just search for TSW and check images, not nice.

Now it's been roughly 2 months since I stopped using it. Seems mostly OK so far, but there's no permanent solution to psoriasis, unfortunately.

I also had a flare up in the groin area for around 2 years, but I felt too embarrassed to mention that. Unfortunately, what I got prescribed specifically mentioned to not use it there. But there's no way I'd be able to mention that to my dermatologist, so after a while of searching I decided on a lower dosage: once a day for 5 days.
It worked perfectly, but I wouldn't recommend it, obviously.