To evaluate the cause of itching you would need a couple of investigations.
In your case since you have been on medications since 3 yrs , drug induced pruritus
also needs to be ruled out. But unless I know the composition of the medicine I cannot comment ion it. Have a word regarding the same with your treating doctor.
1.Complete blood count
2. Absolute eosinophil
4.Renal function test
The reason behind gettine these tests done is that at your age you have to rule out systemic causes of itching as well . Also since you have been on medications for 3 yrs it is always advised to get baseline investigations done to rule out any implication of the medicine on your metabolic parameters.
As for your treatment it is not justified for me to advice without knowing which psychiatric medicines you are taking because at times medicines have drug interactions among themselves.
What I suggest is that once you get the reports see a physician along with your present psychiatric prescription so that after proper clinical evaluation you are prescribed suitable medications.
Usually a course of oral anti histaminics work very well in such cases.