The first rule in trying to figure out what a line of poetry means is to find the end punctuation (in this case, a "period") and back up to the beginning of the thought. Line 12 can't make any sense without seeing how it connects to lines 9-11.

I'm going to take this from the last sestet so that lines 12-14 will fit within the context they came from:

In me thou see'st the glowing of such fire
That on the ashes of his youth doth lie,
As the death-bed whereon it must expire
Consumed with that which it was nourish'd by.
This thou perceivest, which makes thy love more strong,
To love that well which thou must leave ere long

The persona is nearing the winter of his life. What the subject (the person the poet speaks to) perceives is this aging, and the idea that it is the dying fire of the persona's youth.

I read lines 9 and 10

(In me thou see'st the glowing of such fire
That on the ashes of his youth doth lie,)

as saying 'you see in me the glowing of a fire that lies on the ashes of *my* youth' (other readings of line 10 suggest that "his" relates to the person he addresses but this does not make sense to me for two reasons: first, he addresses the listener as "thou"; consequently, the corresponding pronoun should be "thy" (or "your"), not his; second, it makes no logical sense (to me) to given what follows).

It would make sense that the ashes of ones youth would be like a death bed that destroys (consumes) life even as it feeds it. The persona is old and is burning out, and the fire that burns him is what is left of his youthful passion.

My reading of the last two lines is that the person the poet addresses sees everything--the fire and the slow extinguishment of it, but still loves (in fact, loves with even more fervor because he OR she knows the love is marked by impending death):

This thou perceivest, which makes thy love more strong,
To love that well which thou must leave ere long

Line 12, "Consumed with that which it is nourished by" refers to life itself. We are all in the process of dying even as we live.

Line 13: "This thou perceiv'st, which makes thy love more strong." When one realizes the temporal nature of life, one can appreciate its fleeting state. (Perceiv'st = "to see").

Line 14: "To love that well which thou must leave ere long." Again, to love while one is alive is to really live. To understand life is not ours forever helps reinforce how precious each moment can be.