Stanza 2 Summary

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

My tender age in sorrow did begin:

And we're back to Adam. The speaker was born into "sorrow" because he and all other humans inherit Adam's sin, according to the teachings of Christianity that Herbert adhered to. "Tender age" here just means "youth."
Sounds like a pretty raw deal, right? You've done nothing, but as soon as you're born you're slapped with some ancient dead guy's mistakes. No wonder the speaker wants to rise out of the sadness and pain and sin that Adam left us.

And still with sicknesses and shame
Thou didst so punish sin,
That I became
Most thin.

Just like in the first stanza, Herbert follows the downward spiral of human life—only this time, he's describing his own life, not Adam's.
Bad to begin with, life keeps getting worse as God punishes both the speaker's own sins and the sin he inherited from Adam.
As he describes in the poem, accumulated sickness and shame eventually whittle away at the speaker physically and spiritually until at line 15 he becomes "most thin."
Once again the poem's form reflects the content: the lines get littler as Herbert becomes thinner, with lines 15 and 16 petering out into 2 syllables each.
Check out the sibilant bonanza of S's in lines 12-13: "still," "sicknesses" (that's 4 S's in one word), "shame," "didst," "so," "punish," "sin."
Tricky Herbert's at it again. Just like its line 5 twin, line 15 smacks us upside the head with some poetic irony. We've got "most" but then we get "thin," underlining the all-round bummer nature of the situation.

Lines 16-18

With thee
Let me combine,
And feel this day thy victory:

Line 16, an exact repeat of line 6, again marks the turn around the happy corner. The speaker may be sinful, ashamed, and thin, but now things are looking up. The fact that these cheery words are "with thee" emphasizes how central God is to these improvements.
The switch from past tense to the hypothetical future in line 17 ("let me") marks the speaker's transformation from passive victim of the past to awesome flying bird-man of the future.
The speaker's request is essentially the same as the one made in stanza 1, but the word choice and imagery makes it slightly more dramatic. Before, the speaker asked to rise up like larks and sing in worship. Now he asks something more intimate. He wants to "combine" with God and instead of singing his victory, he wants to "feel" it himself. It's clear that the speaker wants to get closer to God and find strength in him, but we're not sure yet what exactly he means by "combine."
Meanwhile, is something getting bigger? Yep, the lines are growing again, expanding with Herbert's hopes.
Nice-lookin' Y lineup there, Herbert. We've got "day," "thy," and "victory." Keeping it pretty or keeping it real? Check out how visual patterns support meaning below in "Symbols, Imagery, Wordplay."

For, if I imp my wing on thine,
Affliction shall advance the flight in me.

The speaker explains his meaning more clearly in line 19, turning again to his feathered friends. This time, however, he uses hawks instead of larks and, instead of just rising with God, he asks for an extra boost: he wants to "imp" his wing to God's.
What the heck is "to imp"? If you keep pet hawks (we don't either, since it's no longer the 17th century), you might know that it's a technical term meaning to repair a damaged feather by attaching part of a new feather. This implies (eh? eh?) that the speaker is too damaged by sin, too thin and sick, to fly properly on his own. He needs some of God's feathers to strengthen him.
In line 20 he reaches a similar conclusion to line 10. Just as Adam's fall increased the distance of his flight in stanza 1, now his own affliction (his sin and God's punishment) lengthens his flight. "Advance" here means "increase."
You've got beautiful I's, Herbert: "if," "I," "imp," "wing," "thine." We've also got two groupings of assonant I's: the long I's of "I," "my," and "thine" and the short I's of "if," "imp," and "wing."
And for more on how all these A's put the "a" in "alliteration" and "assonance," head down to "Symbols, Imagery, Wordplay."