Looking positively it sounds like your symptoms are gradually fading as they should, it's not uncommon for some to linger. It's gauging how much you can do without getting too brain fatigued, basically learning pacing ones self and recognise when to stop. Eventually your stamina will improve. One thing that might be making it harder is your age, you're brain is dealing with hormones and you are still growing plus it's repairing itself.

It's also common to dwell on PCS too much which doesn't help with recovery in my experience. A combination of exercise, rest and nutrition... and laughter is all we can do really.

As a footnote I would become 'head safe' and be mindful situations where you could get another concussion and take precautions, I know with me this will be a life long consideration.