Lines 9-12 Summary

Get out the microscope, because we’re going through this poem line-by-line.

And when I feel, fair creature of an hour!
That I shall never look upon thee more,

Hmmm… let's take a little poll here: who feels excited about being called a "fair creature of an hour"? Anyone? Anyone?
On the positive side, being "fair" is the 19th-century equivalent of being hot. So, that's good. Being the "creature of an hour," though, could mean a couple of things – and we're warning you now, neither of them are good.
For one thing, it could mean that the "hour" is Keats' hour: as in, he likes you now, but tomorrow….well, who knows?
Then again, it could mean that you're mortal, just like him. In other words, in comparison to things like the heavens and the clouds, which measure time in eons, humans measure time in hours. You don't have all that much time to spend on this earth.
See? Neither one is particularly flattering, is it? Unless, of course, you buy into Keats' strange sort of ecstatic hopelessness. In that case, you're good to go.

Never have relish in the faery power
Of unreflecting love; […]

Notice all of the negative constructions stacking up here? It's all the semantic counterpart to Keats' philosophy of negative capability. Want to know what we mean? Check out what we have to say about negative capability in "Symbolism, Imagery, Wordplay."
Once again, emotions (like, say, love) are painted in the most grand and imaginative language possible. Love's a "faery power." Maybe that means it's magical and wonderful and generally amazing – just like fairies. Then again, maybe it means that, just like fairies, love doesn't really exist.
Maybe being thrown into "unreflecting" love is a way to get caught up in the crazy, stupid, exciting thrill of forgetting your better judgment and smooching that girl (or guy) you know you should just leave alone.
But has forgetting your better judgment ever turned out to be a good plan in the long run? That's precisely the quandary that Keats packs into one small line (line 12, in case you were wondering). Is stupid crazy love a good thing? We sure can't say, but that doesn't mean that it's not appealing.