Formal:
* J'ai le cœur brisé. (This is the most literal translation and is quite formal.)
More emotional:
* Je suis effondré(e). (This means "I am crushed" and implies a feeling of being completely broken.)
* Je suis anéanti(e). (This means "I am destroyed" and is very intense.)
* Mon cœur est en miettes. (This means "My heart is in pieces" and is very evocative.)
Less intense:
* Je suis très triste. (This means "I am very sad" and is a more general way to express sadness.)
* Je suis dévasté(e). (This means "I am devastated" and is a more moderate way to express heartbreak.)
You can also use a phrase like "j'ai le cœur lourd" (my heart is heavy), which is more subtle but still conveys a sense of sadness and despair.
Ultimately, the best translation will depend on the context and the level of intensity you want to express.