Very distressing (13)
(Other definitions for heartbreaking that I've seen before include "Pitiful" , "Distressful" , "Tragic" , "Causing great distress" , "Extremely sad" .)
(Other definitions for tragic that I've seen before include "Extremely sad and distressing" , "Very sad; Schubert's 4th symphony nickname" , "Disastrous, desperately sad" , "Terrible" , "Deeply distressing" .)