OMG, your not ranting and this is one of the hadest things to do, explain how you feel to loved ones. Be paitient, keep trying to talk. Dont drum it in await your moments, she will open up eventually. Do you dress at home? not that its that important but I wondering how serious she thinks it may be. If you are serious about this and you are seing a theropist then sit her down to talk, tell her you are serious about this, tell her how you feel and why you have to do this. Be honest with her.
Your mom will be worried about you and wont want you to hurt your self so explain how much you are hurting allready.