2 Timothy 2:8-13: in context

Second Timothy reads like a last letter. Paul is in chains, and he writes to a younger colleague with the plainness of a man who does not expect to write many more. This passage holds the center of it.

Remember Jesus Christ, risen from the dead, descended from David. Then his own situation: for this gospel I suffer to the point of being chained like a criminal. And immediately, the line that turns it: but the word of God is not chained. The man is bound, the message is not, and he says it while the chains are actually on him.

Then he quotes what appears to be an early hymn, four lines in pairs. If we died with him, we will also live with him. If we endure, we will also reign with him. If we deny him, he will also deny us. If we are faithless, he remains faithful, for he cannot deny himself. The first three lines run in symmetry, our action and its answer. The fourth breaks the pattern. The expected line would be that our unfaithfulness ends it. Instead the symmetry snaps, because God's faithfulness is not a response to ours but a property of who he is. Where am I assuming a relationship works on exact reciprocity, and what changes if the last line does not match the first three?