How to Stop Mentally Beating Yourself Up?

Hiya! This post is quite a while ago so I hope you're doing okay. Take some time to yourself to process this and if you struggle to set boundaries, that's okay. Take baby steps. Like others have said, if you have nothing to apologize for then you have nothing to apologize for. Express to your mother-in-law that maybe there are some things you could have rephrased better (I highly doubt it since you stood your ground against her), you're standing by what you said at the end of the day. If she's going out of her way to deliberately sabotage her relationship with you, your husband (HER OWN SON) and her grandchildren simply because you won't apologize then understand that this is the choice that SHE made, not you. She could have gone with different path routes but she chose this instead. That's on her, not you.

I think the main issue with myself is, I never set a boundary and kept it, especially with my mother in law.

If everyone agrees with you, you shouldnt feel bad. Give yourself credit for setting up a boundary.

It seems like you have nothing to be sorry for. You don't have to apologize for other peoples bad attitude.