Here's the thing... People who are abusive prey on your fears. Everything you're afraid of being, they'll say that you are if you leave. You seem like a compassionate person, and I imagine if he hasn't already tried to use that against you, he will if/when you try to leave. I had a partner who threatened to kill himself if I left, so the first three times I tried to leave him, I failed. He made me feel like I was a bad person, like I didn't love him or like I lacked compassion because I didn't want to be with him anymore. As an interesting parallel... He didn't want me to go to college, and that is why I left him.
No matter what the circumstances, you are never (NEVER) obligated to stay in a relationship. You always have the right to choose not to be with someone. You don't owe anyone anything, even if you'd been married for 20 years.
There is never, ever anything wrong with choosing your own health and safety. This is about more than just medical school. Someone who discourages you, degrades you, and dismisses your concerns with laughter, mocking and manipulation is not good for your health or your safety. While I understand deeply, from personal experience, that you will likely feel guilt and shame and regret and fear and other things if you leave... You are not wrong for picking your own side in this. He's not fighting a fair battle. This isn't just a disagreement.
What it comes down to is this... It isn't about your career. It isn't about the MCAT or your studying or whether you're a good girlfriend or how much time you spend together. It's not about his insecurities, and it isn't your job to prove to him that what he's afraid of (being alone, being unlovable, etc, etc) is not true. You aren't responsible for the way he sees the world. None of these things matter, because the WAY that he is talking to you and treating you is abusive.
No one asks to be abused. It doesn't matter if one time you could've spent an hour with him you didn't. It doesn't matter if you feel like you could try harder, or do more. It isn't about priorities anymore. If you feel trapped, which you mentioned you did... It's no longer healthy or safe.
There's nothing wrong with you if you're feeling confused, scared, trapped, or like a bad person for wanting to leave. That is normal in a situation like this. A huge range of emotions is normal here. Please know that I am happy to support you however I can. Feel free to PM even if you aren't ready to leave yet. I understand why you might feel conflicted.