Lines 30-39 Summary

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

Courage was mine, and I had mystery;
Wisdom was mine, and I had mastery;

Speaker number two is looking back at the glory days. At the time, as a soldier, he felt like he was courageous and wise, and he was totally cool with carrying out his duty. Good at it, too.
Now we know it's a different story. He thinks war is a total bust, and now he's spending eternity in the worst place ever—hell.

To miss the march of this retreating world
Into vain citadels that are not walled.

Again, soldiers in war are moving in the wrong direction—the world is "retreating" from them. War destroys so much of the world that as they fight, the previous, real world, or world as they knew it, gets further and further away.
Vocab alert: vain, in this case, means producing no result, and a citadel is a fortress that protects a city. In other words, the soldiers are fighting for nothing—it won't get them anything in the end. And what they thought was their protection, or fortress, isn't. Nothing can protect them from the damage war does.
This guy is in full war-bashing mode. He definitely woke up on the wrong side of the bed. (Is there a right side of the bed in hell, though?)

Then, when much blood had clogged their chariot-wheels
I would go up and wash them from sweet wells,
Even with truths that lie too deep for taint.

"Blood had clogged their chariot-wheels," while a totally awesome and gory image, isn't literal. What speaker number two is trying to get at here is that, at some point, fighting takes its toll, and as a soldier, you're left with an overwhelming amount of "blood on your hands"—or enemy deaths that you have to live with and find a way to deal with somehow. He's being metaphorical.
He's saying he's sympathetic to these guys, and if he could he'd wash the blood from them. Again, not literal. He means he'd wash them of their sins, if he could, so they wouldn't carry the burden of all of those deaths. And he'd try to soothe them with the truth, because the truth is too powerful to be tainted even by the most horrific of battles.
There's that truth again. Remember we first read about it in the beginning of speaker two's speech. Owen is super focused on the importance of beauty and truth, and it really seems like he feels there is no place for either in war.

I would have poured my spirit without stint
But not through wounds; not on the cess of war.
Foreheads of men have bled where no wounds were.

In line 37 he's saying that he would have given it his all. We can assume he means that he would have given life (had he had more of it) his all. "Without stint" just means without restriction. He would have poured all of himself into life.
In line 38 he modifies that statement: he'd do anything with every ounce of his being, except fight in war. He refers to war as a "cess" or curse, and it also makes us think of cesspool or sewer, which is the nastiest of nasty.
When he says, "Foreheads of men have bled where no wounds were" he means that soldiers have suffered mental and emotional anguish just as much as physical pain. And that the mental and emotional anguish doesn't heal easily. He's proof of that. He's still suffering, and the poor guy's dead.