Although, I wonder if I really fucked my brain/habits up so much that I'll never reach my full capacity.
There's no such thing as your "full capacity". What you're doing right now, that is your full capacity. Either accept that you're at your limit or actually do something to prove you're not.
I've been learning to code lately and worked up something like this not too long ago - http://mementomori.neocities.org/. Kind of buggy and unfinished, but it works. Mine uses weeks rather than months.
Frame control; e.g., using and abusing language to empower oneself (or one's group or communities) and disempower outsiders in the pretext of giving advice.
Well, gosh, your life is meaningless drivel unless you fill those boxes with stuff! Produce! Nevermind what your wife and kids think of you, nevermind the laughter of your friends, the beauty you saw, the wonders you imagined. All those times you helped out a bum, cooked dinner, played with your niece: meaningless, if it got in the way of putting an achievement into a month-box! Produce! Produce! What are you going to have to etch on your tombstone if you don't have a dozen github URLs?
I think that's the point. It doesn't matter what you fill them with--fun, work, standing on your head--just that you feel accomplished by having done what you've done. If you don't, then maybe it's time for some change.
There's no such thing as your "full capacity". What you're doing right now, that is your full capacity. Either accept that you're at your limit or actually do something to prove you're not.