I NEED HELP WITH POTTY TRAINING.....BADLY!!!?

MY SON IS JUST ABOUT TO TURN 3 JULY 21ST AND WE HAVE BEEN TRYING TO POTTY TRAIN HIM FOR I WOULD SAY ABOUT 2 OR SO MONTHS. HE HAS THE PEEING DOWN A BIT BUT FIGHTS WHEN WE TELL HIM TO GO. AND HE HAS A HUGH PROBLEM WITH GOING POO. HE ABSOLUTELY REFUSES TO AND WILL CRY HYSTERICALLY TO THE POINT WHERE HE THROWS UP. HE FIGHTS TOOTH AND NAIL AND WE RESULT IN YELLING AT HIM AND SENDING HIM TO BED OUT OF OUR OWN FRUSTRATION. WE KNOW ITS NOT THE RIGHT THING TO DO AND ARE TRYING ARE HARDEST TO DO EVERYTHING IN A POSITIVE WAY. WE ARE AT OUR WITS END AND NEED HELP BADLY...I DONT THINK HE IS READY AND AM AFRAID OF REGRESSING TO DIAPERS UNTIL HE IS, BUT WE ARE TO THAT POINT.I DONT WANT TO MAKE THIS ANY MORE OF A NEGATIVE THING THAN IT HAS BEEN. PLEASE HELP IF U ARE GOING THROUGH OR WEBT THROUGH THIS AND HOW CAN WE WORK THROUGH THIS....THXS A LOT