Your mother is mistaken. Well educated woman find quality partners. It may be a tad harder because the pool of men is smaller but that is no reason to settle.
FWIW I have a big deal terminal degree. I dated lots of men with that same pedigree. It didn't make them better people, BFs or lovers. When I met & married my husband he hadn't even graduated from college & was going to school on line. While I have more sheepskins to hang up that has never adversely effected our relationship. All of my friends from school, both undergrad & grad school are happily married. As long as you don't laud your education over someone's head, your level of education should not be the driving predictor of your love like.
Your BF is nuts to take on more debt without a plan. Whether you stay with him or not, do at least one last motherly thing for him & talk him out of getting a Masters he doesn't know he wants. Encourage him to get a job. Whatever you do, do not pick the school to get your Masters based on where he's going.
Your life is at a transition point right now. You know things will change after graduation & that has you a bit on edge. Change is scary, even good change. Go easier on yourself.
You know in your heart that your BF is not your future husband at least not as things stand. Listen to yourself, not your mom.