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

Approach her in a way that its clear you are explaining something to her and whatever you are going to say is not up for conversation (but its best to do this respectfully). Your intent is to set boundaries with her and it might be best to walk her through your thought process. For example: 'I feel x when you do x and I would appreciate if you could respect this boundary that I have and stop xxx.' Elaborate more if you can do so but don't be afraid to let her know exactly what it is you are feeling

Be honest and friendly. Do not say it when you are angry. Politely ask for some space as you consider her feelings too. Also, try to understand her point of view. Understanding her better could help you get a hang of things and prevent further conflict escalation. I would speak with her, try to understand her, try to know her intentions. Sometimes, when we think someone is wrong, we could be the one doing the wrong thig. So I would analyze the situation. If my mother in law is wrong, I would give her the option of working on her attitude or simpley giving me some space.

Boundaries can be tough especially with in laws. First, I want to say, your feelings are valid even if others aren't respecting them. Part of communicating to others when they offend us is about focusing on what outcome we want out of the situation and not letting the other person force us to concede more than we feel comfortable with. When communicating our feelings, it is helpful to focus on the behavior and why it is upsetting. If you don't feel comfortable talking to them directly it is also okay to write a letter. Sometimes this is useful when trying to communicate with someone who doesn't allow room for you to speak. Having these conversations is tough, I encourage you to be kind to yourself.

What specifically is making you feel like you need space? Is there a reason that you feel she is effecting you negatively. If you haven’t had a conversation with her yet, it’s a good idea to let her now how your feeling and why. When approaching her make sure to be calm and gentle. Come into the conversation with an open mind and a ready to listen attitude. This will make your conversation go a lot more smoothly. When addressing family it’s important to come out of a place of love. Make sure you are understanding and open this will make for a hopefully smoother conversation.

You can always tell her politely by saying "it's our personal issue, we know how to solve it, we don't need your opinion here because it's my family not yours." We are trying to safe our marriage here so please don't add up more to it. Seat and confronting this mother in law of yours might help her to get closure on what she's doing right now, I understand clearly she worried about her kid's but interfering in their personal family issue are over her limits, it may make the situation even more worst than solving it. If this technique don't work than i don't know how else to fix your marriage.