With torn and bleeding hearts we smile, (4)
"Torn and bleeding hearts" aren't just catchy clichés for love songs. They also point to emotional conflicts and the fact that there are a lot of different factors pulling at the person experiencing them. And the "we" tells us that he's not suffering alone.
Why should the world be over-wise, In counting all our tears and sighs? (6-7)
There are too many tears and sighs to "count" here, and yet the world still isn't savvy to what's really going on. So, not only is there a ton of pain being felt, but the world's not even paying attention, which makes the speaker's suffering worse.
We smile, but, O great Christ, our cries To thee from tortured souls arise. (10-11)
The "O" here emphasizes the severity of the speaker's suffering in a simple and clear way. Not to mention the inclusion of "tortured souls," which indicates that this kind of suffering goes far beyond the physical world and into the metaphysical and spiritual realm. Ouch.