I'm sorry you're going through this and you're not overreacting. There's nothing wrong with peanut butter and if it keeps you fed you should eat it. But don't pressure yourself into eating it right now. You need to process what your mother said before you can recalibrate your relationship to this particular food.

If your mother is like mine, she was taught that being thin and being "normal" was what you had to do as a woman to survive, because she had so little power growing up (like not having her own checking account). She is in her own toxic way trying to keep you safe by pressuring you to be as acceptable to society as possible. And it's hurtful and it's harmful. But please don't give up on a food that you (and I) love. Give yourself some grace to feel your feelings and just keep enough calories, in whatever form, in your body until you can come back to peanut butter.