As the death-bed whereon it must expire,Consumed with that which it was nourished by. (11-12)
Following up on the previous two lines, which convey the idea of the speaker's ongoing vitality, these lines could be interpreted as the speaker saying "Hey, I've still got it, but I don't have that much time left—so let's get it on!" What do you think?
This thou perceiv'st, which makes thy love more strong, (13)
If you weren't convinced by our interpretation of the previous line, just check out what's happening here: the speaker says that seeing all the horrible stuff that is happening to him will make the listener's "love more strong." How can that be? Well, as we talk about in the "Symbols, Imagery, Wordplay" section of this module, we think this line is supposed to be a paradox. So some confusion on the reader's part is exactly what Shakespeare might be going for here. That sets us up for the payoff in the next line, where the speaker will let us know exactly what he means.
To love that well which thou must leave ere long. (14)
There you have it. Whether or not you see the final twist in the poem as foreshadowed earlier on, the final idea is clear: the speaker thinks that knowing their love can't last forever will make the listener love him all the more. Does that sound like a convincing argument to you?