How can I tell my mother in law to back off?

With calm, try to be kind and say something like "I would really appreciate if you could respect my space". Making eye contact is always a better.

Say these words, " I appreciate you. You've raised an amazing child and I'm grateful to have you in my life. I feel that through marriage you haven't lost a child you've just gained me as part of your family. I feel humbled to be called your son (or daughter ) -in -law and I've gained a mother through this union. You've raised an amazing child and I want you to relax and back off a little bit to see the amazing child you've raised and to trust me to. Give us a little space to set boundaries in our relation, so we can improve our relationship."

Explain to her that her son is very important to you. That you need her to give you two space and you would really appreciate it.

Try asking her what's wrong or if you did something wrong. Try asking nicely and talk to her personally

You need to speak respectfully to her, something along the lines of "i appreciate your help and siggestions, but this is something for ntself & spouse to deal sith by ourselves.

Firstly, say something positive, such as her daughter/son is lucky to have such a doting mother. Then tell her that you are concerned that she seems to be overly worried about how you are handling your family. Ask her what concerns her that makes her feel she must be so involved. Try to assure her that you are equally loving and supportive of your family and have hopes that she will trust you and feel free to spend more time with her favorite hobbies, etc.

I would basically have a calm relaxed chat with her on what's bothering you in a positive calm manner.

You could try telling her that you need some space. She might not understand that you can handle things on your own.

Try something nicer and don't be too rude or you'll destroy your relationship with her. Maybe you'll find a common language with her after all

behave nicely and then tell her that you can do it by yourself.. if she doesn't back off talk with her in an open manner.