Reassure her you are the same person you were before and will be come a better person because you won't have to hid this problem anymore. Almost all of us improve as we get treatment because we no longer have to deal with the depression and uncomfortable feelings any more. My personality is a great deal different that before I started treatment and I am sure if you knew the old me and the new me, you would like the new me much better.
A little side note, your mother will have to do a lot to help you, so help her when ever you can and thank her often for what she is doing. This will help her understand how important this is to you.