As a mum myself, I would want you to tell me rather than find out another way. I understand that it is not easy but once you tell her she will be able to support and help you, this does not necessarily mean that you will lose control of things, believe me I do not feel in control of my daughters SH at all! I found out about my daughter by seeing her marks and what upset me the most was feeling that she felt she couldnt tell me. Obviously I dont know your mum but I'm sure she will appreciate your honesty and bravery in telling her.
Here if you want to talk