My scalp is dry and itchy i've always had dandruff and get my hair bleached. what can be done? anti dandruff shampoo helps but still itchy

Details matter: You neglected to identify WHICH dandruff shampoo was tried. There are several different kinds. You also need to learn how to get the best results by having the shampoo contact the scalp long enough before rinsing. Hydrocortisone in a liquid can be applied. Prescription strength usually stops the itch and the flaking. No antihistamine will help this kind of itch.