The abstract noun "faithfulness" can be translated as an adjective. Alternate translation: "I have trusted that you are faithful to your covenant" or "I have trusted you because you are faithful to your covenant" (See: [[rc://en/ta/man/jit/figs-abstractnouns]])
Here "my heart" represents the whole person. Alternate translation: "I will rejoice because you have rescued me" (See: [[rc://en/ta/man/jit/figs-synecdoche]])