These two phrases are parallel. The writer speaks of loyalty to God as if it were following him, and disloyalty as if it were turning away from him. (See: [[rc://en/ta/man/jit/figs-parallelism]] and [[rc://en/ta/man/jit/figs-metaphor]])
Here the word "heart" refers to the emotions, and specifically to loyalty and devotion. Alternate translation: "We have not stopped being loyal to you" (See: [[rc://en/ta/man/jit/figs-metonymy]])