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.
Others self-harm by cutting because they feel numb and the pain of self-harm allows them to feel something. Some even report that cutting provides an energy rush, although this is uncommon. Cutting also allows a person to express pain when they don't know any other way to express it.