Is this poem actually cheerful?

Yes, yes it is actually; it's very beautiful as well. I'm not well read on Emily inson and I'm not trying to compare this to one of hers, but for some reason she came to my mind after reading this. I guess when I think of Emily's writing I think of soft, eloquent, beauty--that's what I experienced with this poem (come to think of it, most of your poetry does that to me). This is the very definition of a Sonnet and that ending couplet is timeless and true.