Why does the heart remain attached to a girl after the relationship ends, and does God answer prayers from a heart like this?

You must strive to rid yourself of attachment to images by contemplating their deficiencies, ephemerality, and flaws. Strive to attach your heart solely to Allah, by reflecting on His perfect attributes and blessings, by reciting the Quran with contemplation, by accompanying the righteous, and by adhering to remembrance (dhikr) and supplication (du'a). If you do that, along with having a good opinion of Allah, it is hoped that the blameworthy attachment will be removed from your heart. And trust in Allah and rely on Him for the acceptance of your supplication.