All good advice here! I'm so sorry you are feeling so much dread and I really recognise that in relation to my own mother.
It is getting easier now,but I am still practising ,and may go NC one day yet.
Try have a long ,sympathetic talk to yourself before she arrives .
Firstly,it is ok to cry! You sound like a strong, successful woman.
Those of us who have not had the best parenting- particularly from our mothers ,often cope by trying to make everything perfect .That's ok ,too.It would definitely be worth having some counselling in the future to work through some of the past stuff,and look at different ways to deal with it,now.But for this visit,try to tell yourself that you are not responsible for any of the failings in your relationship with her,nor are you responsible for how she feels,or for failing to measure up to whatever version of you she would like you to be.
Try not to allow in any feelings of guilt regarding your feelings towards your mother.Be very gentle on yourself.Build an invisible wall around yourself with bricks made out of all your strengths - good mum who places a high value on family,successful professional woman,etc - and refuse to allow these (and others- think of them,how do your friends,colleagues etc see you?) to be dismantled by the barbed remarks your mother makes.
You say you will "feel like a small child again".Could that be because when you were a small child,she did not give you - emotionally - what you needed? Well,try to be thoughtful towards that part of yourself while she is around.It is hard being mummy when our mothers behaviour causes us to regress like this,and it takes practice and support to change this;so while your mother is around,allow for this.Try to drop your standards a bit,or at least don't give yourself a hard time if they slip.
Make time to have time out.Think of how you might deal with a work colleague/client/boss who was really irritating/offensive.You'd have to maintain a professional stance,yet keep an emotional distance.But also take a lot of breaks.
Have treats available in advance.I keep chocolate and,I have to say,my favourite whiskey,upstairs in my room.My m never stays overnight,but during an afternoon,I have been known to take my work phone upstairs to "answer a call!" The added dimension of secret,naughty behaviour must fit with my barely conquered regression!!
Have realistic expectations.You are expecting her to be hard work,but it is still hard to be faced with the reality of a less than ideal mother.Keep reminding yourself that you have the honourable intention of enabling a relationship between your dc and your m,but that she may well not be up to this.What would your bottom line be,before you give up and decide you are all better off without her at all.
Be strong! Good luck and come back and post when she's gone - or,if you're anything like me,run upstairs and post while she's with you,we'll be here.