These kinds of problems can sneak up on you over time, and I wouldn't be surprised if they were directly connected to that "simple bump on the head".

Are you getting enough rest? Your list "long and short-term memory problems, difficulty focusing, difficulty completing tasks, decreased impulse control (including problems with my temper), and difficulty learning new tasks" are exactly what happen to me if I am over-tired (let's be honest - exhausted), over the long term. Being tired makes everything worse, especially behavior issues.

Are you drinking enough water and eating properly, as well? Your brain demands a lot to keep going at peak - even an ageing brain (which mine is, too)

When I'm worn out, my motivation goes through the floor, and I have no interest in dealing with any living being in person. Online is fine, but I honestly can't be bothered with real live interactions.

Check your sleep and food and water intake. Making some minor adjustments and staying steady with them might help.

Good luck.