This can have several means, in the context of the song, it could mean that she's better at watching her back, she's already had a fight with her heart to leave the person in the song, so it'll be easier to win the fight next time or it could mean that because she's already had the fight, and she's won that fight to stay away from the man that's hurt her, she's lighter on her feet because that weight of being in a bad relationship is gone from her shoulders