Congratulations on medical school graduation! You have such great advice on this board. I would approach this in two ways.
First and foremost, focus on celebrating you. What does that look like? A trip, a special gift to self to celebrate yourself, a day of pampering? The road to medical school graduation is a long and hard one and just like your friends, you have earned a special celebration. Since your mom is unlikely to plan a meaningful celebration, define and plan something for yourself. When we allow ourselves to celebrate ourselves we get exactly what is most meaningful to us. Give yourself permission to celebrate you. (Kudos on the next day boundary also).
Second, let go of any expectations of your mom to support you. Maybe expect her to be self involved, self centered and to create drama. If you let go of positive (maybe unrealistic) expectations she can’t disappoint you. If she is just a little self centered, shows up, but makes it about her self but no major drama, then it is a win. And if it is worst case scenario, you are mentally prepared. I am so sorry you don’t have the family support you deserve, but know we are all cheering you on this special day.
guarantee you that someone else at your event has something similar happening with their families.
All families have some level of dysfunction, and milestone celebrations tend to surface and amplify dysfunction. It is often hard to see as there is a lot of “covering” that occurs. But my guess is that even within your close friend group someone is struggling with difficult family dynamics. You are not alone.
The best part of your situation is your planned move 5 hours away! You will be very busy with your post graduate residency training and building your next personal and professional chapter. That will make the needed boundaries a lot easier and is part of building the next part of your life away from her.
Congratulations Petoniscat!

Your future is bright. Do not let her ruin a very special day!