Both unilateral and bilateral contracts are enforceable in court. For example, a unilateral contract is enforceable when someone chooses to begin fulfilling the act demanded by the promisor. A bilateral contract is enforceable from the get-go; both parties are bound the promise.
Yes, scratching is a form of self harm. Try to tackle this problem while you can because it has the potential to get worse. I started out scratching and now I burn myself with a lighter and cut myself. It's a self-destructive way to deal with overwhelming mental/emotional pain.
Scratching may cause breaks in the skin, bleeding, and infection. If your skin feels itchy, let your doctor know so it can be treated and relieved. The way pruritus feels and how long it lasts is not the same in everyone. The skin is the largest organ of the body.