I went through a lot a few years ago and thought I was over it until this past October I started slipping again. Didn't know how to tell anyone. Even my best friend, I just couldn't bring myself to do it. So, like you, I decided to just wait till someone asked me. Well, BAD idea. Finally I got back into counseling and made some other changes...but I didn't do enough soon enough. Had I told someone and gone back into counseling back in October, then I would not have had the major depressive episode that I had about a month ago. I was in a fog, and couldn't trust myself, and basically did some pretty stupid things that put me back in the hospital.

Maybe try writing out what your going to say, and tell a friend. Then have that friend help you figure out what to say to your mom. If all else fails, you're allowed to go into counseling without your parent's knowledge once you're 14 (I don't know how old you are).
If you go to a church, try reaching out to a pastor, youth pastor, rabbi, priest, etc. They can be really understanding and quite helpful. You could try going to a school counselor and telling them. You could also call a crisis line (you don't have to be in a crisis to call one) and ask one of those people for advice on what to do. Those people are also really helpful.

But no matter what you do...do NOT keep quiet. That is the worst thing you can do. I know it's not easy, but you are strong enough! You can do this! Write that on your hand so that when the moment comes and your heart is pounding in your throat, you're sweating like mad and you want to cry and hide in a corner, you will remember the truth. Because yes, yes, my dear, you CAN do this!!