I’m so sorry for the loss of your first baby. It must have been very hard to deal with your own grief and with your bf’s mother’s omments. You are very mature to continue to work at making a relationship with her in spite of it all. Congratulations on this pregnancy. I hope this baby brings you joy.
Meanwhile, it sounds to me like your future mother-in-law hasn’t accepted that her son is an adult. Instead of taking pride and pleasure in his finding someone who loves him and making a family, she would like to keep him stuck as her little boy. How sad for her that she can’t participate in the joy of a coming grandchild. How hard for your bf that he’s caught between you.
The most loving thing you can do for him is to drop your end of the argument. Yes, you are in an argument. All the work you are doing to try to get him to see your point of view and to change his mother’s opinion of you is your part of the fight. Instead, let him know that as much as you would like his mother’s friendship and his family’s support, you can’t make them do something they don’t want to do.
Don’t ever speak badly about his mother or his family to him. You don’t need to. The contrast with how your family behaves is obvious. Whenever his mom is around, be polite. You don’t need to take in her negativity. There is no need to compete.You’re the one your bf comes home to every night. You’re the one he is making a new family with. If you always welcome his family members when you see them, you can hope they will one day feel kind of silly for keeping up the distance. If you are always courteous and open to having a relationship with your bf’s mom, she might eventually come around. If not, it’s definitely her loss.
I wish you well.