I mean, people experience a lot of things, some more normal than others, but even that doesn't mean they aren't related to your injury.
Being tired, lacking academic drive, etc is definitely a normal part of life for a lot of people your age, that said brain injuries can have a pretty major impact on everything.
If you want to see a doctor, and have health insurance or can afford to see one, go. Who cares if they say "you should have some before". It doesn't mean anything. You didn't, it's in the past, but that's not a good reason at all for not going. If you're concerned, you're having problems, and you want to know what's up go for it. They might not be able to tell you much, but you might be able to find out some things as well as getting advice on what can help you heal up etc/
Also, if you're worried about a head injury, I wouldn't start a half assed, self prescribed drug regimen in total yolo mode. I'm not saying it's going to do anything bad, but you broke your head, you're having issues that are messing with your life, you're not sure whether or not they stem from the head injury, I would definitely be careful/research anything you do.
"The mountains are calling and I must go."
- John Muir