Antonyms for the phrase "break one's heart" could include words that communicate love and affection, such as "mend one's heart" or "heal one's heart." Comforting words such as "soothe" or "calm" could also be used as antonyms.
Other antonyms could include phrases describing positive experiences, like "give one joy" or "fill one's heart with happiness." Words that engender feelings of hope, such as "restore," "build up," and "strengthen," could also be fitting antonyms.
Ultimately, the antonyms for "break one's heart" all communicate the idea of uplifting, healing, and positive emotional experiences that lift us up instead of tearing us down.