Alexandra,
You don't really have to use "كسر", because we have other suitable ways to say
"you/ it broke my heart" in Arabic, such as "فَطَر". I don't mean that using كسر is
wrong, but in Arabic you have many equivalents to the broken heart but not
necessarily using the literal translation, which is كسر in this case.
So if you want to say:
1. "It broke my heart": لقد فَطَرَ/ فَطَرَتْ قلبي Or فَطَرَ/ فَطَرَتْ قلبي
Examples:
The end of the novel broke my heart: نهاية الرواية فَطَرَت قلبي/ فَطَرَت نهاية الرواية قلبي
The sight of the ship sinking broke my heart: منظر السفينة وهي تغرق فَطَرَ قلبي
2. "You broke my heart":
- To a male: فَطَرْتَ قلبي/ لقد فَطَرْتَ قلبي
- To a female: فَطَرْتِ قلبي/ لقد فَطَرْتِ قلبي
3. "Broken heart": قلبٌ مفطور.
If for any reason you still wish to use the verb كسر, then "It broke my heart" would be:
كَسَرَ/ كَسَرَتْ قلبي Or لقد كَسَرَ/ كَسَرَتْ قلبي.
Always give us a context in your questions, this way we'll be able to help you in a better way.
I wish you all the best :)