You are taking the most extreme and rare examples (stories) and making them into the norm. No, you will not get a concussion hitting your head on a frig door. There is not enough mass. Other things must come into play. The skin sensation of pain or discomfort could easily trigger a reaction. If the impact was a concussion, you would immediately have at least a minor set of concussion sensations. As you described, you did not.

Your morning tension headaches are likely from getting lousy sleep. Your last concussion likely has left you with a neck that is sensitive to sleeping position. Poor blood flow during sleep can cause all kinds of struggles. Cervical inflammation from poor position can be a big problem.

You are making quite an anxious jump from hitting your head on the frig door to losing all short term and long term memory function. If you survived your last concussion and scored enough on your MCATs to get accepted,

If I were you, I'd be more concerned about getting control over your anxiety. Med school will likely push any anxiety into over-drive. Poor sleep leaves your brain struggling in a myriad of ways, anxiety, concentration, processing, executive functioning, etc. These will combine to stress you to beyond anything you ever expected.

I'm surprised any doctor will even entertain your concerns about these minor head bumps. Many would be tempted to prescribe anti-anxiety drugs or such to help you get past such thought.

You may need some head touch desensitization training to reteach your brain to not respond to every touch. We teach our brains how to respond to various situations. Cognitive behavior therapy (CBT) can be helpful, too.