You immediately master that skill but completely forget everything you learnt about the last skill you learnt. This means that from the day you acquired this power, you are unable to pick up more than one extra skill from the skill set you had on that life changing day.

So lets say after high school, you fell in a toxic pit of brains (haha it's a power acquiring origin story, why does it have to make sense :P) Then after that day, you are able to pick up any, skill, any knowledge instantly and perfectly but only the one because once you pick up the next you forget the one before. So you will be your high school self + one other skill/ piece of knowledge for the rest of your life.

Still can be pretty good, but good luck with multitasking :P