I'm super glad you posted here to talk it through. There seems to be some serious infantalising, boundary stomping, guilt tripping, shaming, co-dependant nasty shit going on from your mother.

What options do you have for some support? Can you get out of the house for a few hours? Can you talk about it with someone you trust? Would you take a look at r/raisedbynarcissists or r/justnomil or r/momforaminute to get a feel for her behaviour being totally out of line?

Your mother is completely out of line. It's completely natural to masturbate at 17. I started at 14 (I think). She is really overstepping the line trying to shame you like that. I am so sorry for you. All my hugs x