This has been going on for 5 years? I think if it was going to change, it would have done so already. You are right to be concerned; especially if you are thinking about adding children to the situation. I doubt the mom’s treatment of you is about you personally.
It looks like your partner has accepted and participated in a relationship of disrespect. He is discouraged about changing it and is expecting you to join him in that discouragement.
You only have a few choices: You can accept the situation as is. You can try to have a discussion with your partner and mother about what you are and are not willing to accept. Or you can refuse to participate and absent yourself (and your children) from gatherings at his mother’s home.
To me, it’s always worth a try to change things. Start with your partner. Be as frank with him as you have been in your letter. If he doesn’t know how to talk to his mom, suggest that the two of you see a couples therapist, not because the two of you need therapy but because you need some help deciding how to best work towards changing the dynamics when you visit his mom. It’s likely that a therapist who can hear the whole story will have some insights into the mother’s behavior and can offer some suggestions for how to handle it.
I wish you well.