Am so sorry you have a mother like this. What people do to each other, like this, never fails to horrify me. There seem to be no lengths to which some people will not go to make themselves feel superior, validated and vindicated when, in truth they are the perpetrator.
I would suggest that you return the letter, unopened and, obviously, unread. Whatever is in it is irrelevant. Whether your parenting is wrong, your house is messy or you are overweight - it doesn't matter. The letter has one aim and that is to demean you, to criticise and put you down and make you feel less than.
Ask yourself - who is she to criticise you? Was she the perfect mother? No. Has she succeeded in raising her children without fault? No. You have suffered because of her parenting and you continue to suffer.
What has already happened is something you cannot change. It is set. However, your present and definitely you future are yours to determine. You decide how you parent, how you clean, what size you are. Your life is your own, don't let her determine how you live.
I say this as someone who has a highly toxic sister and a mentally ill MIL, both of whom never fail to tell me where I have gone wrong, failed them, taken the wrong decision etc etc. I have ended up in counselling to try to overcome very low self esteem. I am, thankfully, receiving help and am coming out of such damaging relationships. Please, remove yourself from this woman - she will not bring you peace, joy, calm or support.
I am thinking of you and sending you {{{{{hugs}}}}