Jail has broken Dr. Conrad Murray ... physically and mentally ... this according to The Doc himself, who's been calling everyone he knows HYSTERICALLY CRYING and complaining that he's in "constant pain."
TMZ has obtained one of the voicemails Murray recently left for a friend ... in which he sobs uncontrollably and begs for someone to save him.
During the message, Murray says, "I can't sleep now ... it's very hard ... I'm in constant pain ... can someone please help me please!?"
Read more at TMZ