Paradox

In "Batter my heart, three-personed God" by John Donne, a paradox is used in the first quatrain that is used to describe God. In the paradox, the speaker asks God to batter his heart, but then says that He "seeks to mend." How can one "batter" a heart and be called a mendor at the same time? I believe that this adds to the speakers attitude towards God. The speaker seems to get frustrated but then turns to God for help. The speaker also says that he is "betrothed unto your enemy." I took this to mean the devil. The speaker is a sinner, but he goes to God for repentence.