A: Understaby you're feeling betrayed by your formerly "amicable" M.I.L. You are taking some good steps: avoiding touchy subjects when she is drinking, and not relying on her as an outlet for venting about your husband. (Never a good idea.) It seems likely that her behavior actually may be due to a drinking problem. Perhaps she hasn't apologized because she doesn't remember the incident the way you do, or remember it at all. If this is the case, her behavior won't change unless the drinking stops. Getting to the bottom of that will be a family effort. In the meantime, if you want to try to smooth things over, discuss the situation with her by stating facts (not accusations) openly and calmly. Make sure your thoughts are collected, and that she is sober and receptive.
It is hurtful when you feel your partner doesn't back or protect you. There could be any number of reasons why he won't get in the middle. Maybe he feels it ultimately could create a bigger division between the two of you. Maybe having a relationship with his mother requires that he overlook her faults and stay quiet on some subjects. Maybe the behavior that is hurtful to you simply isn't hurtful to him. Who knows what can of worms might be opened if he "comes to your defense." You need to establish boundaries with your mother-in-law to protect yourself, whether or not your husband intervenes.
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Your letter also hints at resentment toward your husband for reasons beyond this incident. Think about whether this is the case, and be careful not to let this situation become a stand-in for other unresolved conflicts. Addressing issues with your husband should be your first priority—and those with your mother-in-law your second.