As a parent, if you ask me “what is the one thing you want for your child” my answer without any reservations is “that my child go to heaven”.
I know that culture says I am supposed to answer “that my child is happy”, but, I know that happiness in this world is temporary, that happy is a fleeting emotion. I know that in leading a life of Christian virtue comes joy.
Desire to see your children and grandchildren go to heaven is not license to be mean or rude. Maybe if you acknowledge “mom, I know that your desire is for me to go to heaven. That is also my desire and my goal in this life. I promise you that the faith foundation you gave me will help me through eternity. I value my faith.” It might calm her down a little bit.
Hiding, sneaking around, lying to her, only is going to make the whole thing worse. You are a grown woman. If you want to go visit your boyfriend, unless you are asking mom to pay for the plane tickets, it is nothing to hide.
Your boyfriend is a grown man. I’d wager that your mom is not the most imposing character he has ever met. If he is a man of virtue, he will be kind and gentlemanly to her in spite of her quirks.
You can search the eleventy million threads here about mixed marriages and read the statistics, the disparity of faith is really a separate thread.