Monster-in-law

In my own dealings with my legitimately borderline personality disorder mother in law (who also had a difficult life, no current husband, and he is one of few things she has) is sometimes it's your fiance's turn to deal with this. While I have had good periods with her sometimes she flips out over the smallest thing then suddenly I am the "white devil" and trying to steal her son.

Have your fiance talk to his mother. How much he loves you and marrying you makes him a husband but does not make him any less of her son. You keep playing the high ground and being the better person in all this, never sink to her level because it will validate her paranoia.

There is nothing that you, personally, can do. It sounds like she requires some therapy and boundary setting. Sometimes it's hard to accept that there's nothing we can do to force someone to like us, but some people aren't happy with their own lives and they take that out on other people.

You cannot change other people. Allow her to feel her feelings, but set boundaries for how much/when/where you will interact with her.

My ex-mother-in-law was like this. She's a narcissist (really) and it had to do with her child not being at her beck and call. Seriously...she used to call him to come take our her trash and my ex-husband cannot say no. It never got any better. Set your boundaries and try not to take things personally, but I hope things get better for you.