Lines 9-13 Summary

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

Of the last bitter hour come like a blight
Over thy spirit, and sad images

Still, this guy isn’t just having any old depressing thoughts. He’s really worried about death ("the last bitter hour").
These thoughts about death come like a plague or disease (a "blight") on his spirit. (By the way, "blight" is a pretty good word. It's often used when referring to diseases plants get. Check out these pictures of potato blight for an example. Can you imagine this guy's spirit getting potato blight? Nasty.)
Did you see what happened there? The speaker of the poem isn’t just talking about a random lover of nature now (the "him" from line 1). All of a sudden, he’s talking about you. Here, for the first time, in line 10, he talks about "thy" (your) spirit. The poem has switched from musing about nature to giving you advice.

Of the stern agony, and shroud, and pall,

We're pretty sure everyone has thought about what it will be like to die. You have, haven't you? Here the speaker gives us some strong images of those scary thoughts.
He talks about the "stern agony" of dying, which we think is a great phrase. Death doesn’t just hurt, it hurts in a sharp, severe, serious way. We think of mean substitute teachers and hall monitors as being "stern," but usually those guys don't cause agony. Death does, though.
The speaker also uses a couple of useful, death-related words, so we’ll break those down for you.
A "shroud" is the cloth you use to wrap up a dead body.
"Pall" is another good, spooky death word. It can mean a cloth that covers a coffin, or it can mean the coffin itself (like when people talk about "pallbearers" at a funeral).

And breathless darkness, and the narrow house,

Here we get some more death imagery, only this time even scarier.
The speaker helps us imagine the "breathless darkness" of the grave and the "narrow house" of the coffin. These lines are really claustrophobic, aren’t they? They make us feel like we’re trapped in some suffocating prison. We're feeling a bit panicky now.

Make thee to shudder, and grow sick at heart;—

The speaker is definitely building a mood here. He wants us to think about those moments where we worry so much about death that we "shudder" and "grow sick at heart."
Soak up this scary feeling, because he’s about to change things up on us.