Sadly, a conventional MRI is not sensitive enough to show damage to the brain's tiny axons. The best way to access your brain injury is to undergo neuropsychological testing. You might want to ask your doctor for a referral.

I agree with the others that your symptoms and mechanism of injury make PCS a probable diagnosis. Fortunately, most patients make good recoveries. You are quite early in the process and, as many people here will tell you, recovery from TBI is rarely a straightforward trajectory of constant improvement. Sometimes it can feel like two steps forward, one step back. So try not to despair when you have a bad patch.

Welcome and good luck.