How does one say:
Knowing that I hurt you (your feelings) is breaking my heart.
"Breaking my heart" I think I know:
胸が張り裂けています。
but the first part and how to connect the two I'm clueless about.
I think the sentence "Knowing that I hurt you " can be translated as あなたを傷つけたと知る.
I translate your sentence as あなたを傷つけたと知り、私の胸は張り裂けそうだ.