The speaker is talking to someone who left them and now wants to comfort them with kind words. The speaker refuses to listen. They call those words lies and say that being told "I still care" only reminds them they were never the real thing, just a "would've been" and a "should've been." The leaving feels physical, like a wound that needs sewing shut, and it has knocked away their confidence.
The mood is raw, bitter and hopeless. The lines about being turned away at heaven and locked out of hell make the speaker sound stranded, with nowhere to go and no one who helped. The ending is harder to pin down. The image of shouting underwater suggests that crying out only hurts more, and the closing advice to "shut your eyes" and "save the peace" can be read as a plea to let it end quietly, or as a darker wish to stop feeling anything at all.