I am so sorry for your loss.
I agree with Doris Day, thje 2 of you need to go to marriage counseling You have been a couple long enough, even without marriage, that this counseling will be a huge help.
Write down the things that seem to come up over and over.
These are just examples...
His mothers interference. Her comments and assumptions about you.,
His lack of standing up for you.
He shuts down at your concerns.
Your loss of the baby and your feelings of loss.
What you are expecting if you get married.
How you want to change.
What are your ultimate goals in marriage.
How you view marriage vs, just being a couple..
If you think you are having problems now, getting married and being stuck..., I mean staying together forever.. can really be hell, I mean wonderful IF you are on the same page. (hee, hee)
FYI, I have been married to my middle school sweetheart for over 30 years. His mother has never been a fan of me.. I no longer speak to her.. And she has learned it is her loss. Because my husband has always been aware of her treatment of me and told her, he would always love me and nothing she could do was ever going to change this.
My husband and I went to marriage counseling early in our marriage and it taught us how t communicate with each other. It is a skill and with guidance you 2 can also learn.