Keats is well-known for his affinity with and appreciation of nature and his poetry is the expression of this. There is a wistful and sad tone to this poem as Keats is aware of his impending death (from tuberculosis) and has a great desire to preserve what he has and have some consistency and allow only indiscernable change. The stars make it seem almost possible to 

live within such a moment for the rest of eternity

although to do so would actually deny the speaker the joys of living. Keats reveals a(n)

harmonious acceptance of nature for what it is, beyond the selfs interpretation of it.

Bright start, if only I were as resolute as you are

But not alone in the darkness and beyond anyone's grasp(aloft)

And being watchful, and fully aware of all around but at the same time removed from it

And like Eremite ( a religious recluse) saving and savoring the moment (almost hiding it away) without even being aware of the passage of time (patient)

Being a part of the process of cleansing with all its cathartic (liberating) properties and the wonder of apparent changes ("the soft-fallen mask") changing the appearance of an  otherwise harsh environment (the mountains and the moors).

Let me experience this but in a manner that allows me to  "feel" (rather than watch as the star has in the previous 8 lines) comfort (pillow'd) and nurturing protection (love's ripening breast) 

That this moment would never end and her breath (symbolic of life itself) to be audible but knowing it must end ("sweet unrest") and if so - if not to live in this moment forever- then to die in a moment of extreme bliss.

The contradictory ending(paradoxical) shows his confusion as he knows that this perfect moment cannot be sustained and life must go on and he will have to face the consequences. Hence the wistful tone...if only...