I have a lot of dandruff. I've tried all shampoos and even cut my hair 1cm long. What can I do?

This condition is... ...Called seborrheic dermatitis, and i'm an expert, because i've had it for many years. Effective otc shampoos all contain either tar, salicylic acid, zinc pyrithione, selenium sulfide, or a combination of them, so look for those ingredients. If the flaking persists, a dermatologist can give you a stronger, prescription shampoo and a foam or lotion to apply to your scalp.
Psoriasis? You might have scalp psoriasis - try t-gel shampoo for a while and if that does not help see a dermatologist. Sometimes topical calcipotriene helps/.