He 'WAS' my best friend, If he might have a year or two left?
If he ain't dead yet, simply keep on spending some more time, talk some more and play some more video games together.
Having lost multiple people to lung conditions myself, everything you just said you're experiencing, it certainly is grief, even though your friend may not be gone just yet. And the whole powerlessness of the situation can easily get you several panic attacks.
I also think/wished I could've done more, back then and still do now. Even if I would have done more, or better, I'd probably still feel the same today.
stuckathome
*hugs*