To get your sweater wearable, first you will need to figure out what is making it itchy. Likely, it is a wool sweater, and if this is the case, there is not much you can do, except maybe wear a T-shirt underneath. If the sweater is machine washable, you can try adding a powdered water softener to the wash. If you need to hand wash it, try washing it with a protein hair shampoo. If the scratchiness is caused from detergent residue, try adding white vinegar (1/2 cup) to the rinse cycle.