So your mother-in-law is ruining your marriage you say. Well, this can be a tricky situation. The storybook idea of marriage has it that when you get married you forsake all others for your husband or wife; but the reality of marriage is that this isn’t what happens in most cases; and why should it? Assuming you’re a woman, your husband’s mother came before you. Without her you would not have him and by extension her to complain about how she’s ruining your life. Your husband is a grown man who can speak up for himself and if his mother is a problem in his marriage it’s his responsibility to talk to his mother about it. If he’s not trying to keep his mother out of his life then he’s apparently fine with her being involved in his life. If you don’t want her to be part of your life then you need to figure out if leaving might be your best option because a man can have any number of wives but only one mother, so to ask him to become estranged from his mother for the sake of your comfort is no more reasonable than if he asked you to become estranged from yours for the sake of his. Any estrangement from his mother should be a choice he makes independently.
Mother-in-laws are like mothers. They generally mean well if even they sometimes step out of line and irritate us. They generally only want the best for their child and they aren’t usually going to do anything to intentionally cause problems for their child.
Why don’t you try to build a relationship with your mother-in-law if you truly want to save your marriage. That is as reasonable an alternative as leaving your husband because his mother is too involved in his life. In fact, it’s arguably a better alternative because it results in many people being happy instead of many people being unhappy because of the break-up of a family.