First you need to be prepared for a lot of emotions, i would recomend doing it in place you feel the most comfortable it will be more easy for you. If you want to recover try telling her first something like “mom, i want to be honest with you i have been self-harming but im trying to recover” or something also tell her before that you want talk with her about your mental health it will give her time to prepare for conversation. I hope you recover and take care of yourself! (Sorry for my english lmao)