The pilot of the heavily loaded Cessna 172 was flying a full-flap VFR approach, when he attempted to execute a go-around at the midpoint of the 2,500 foot runway. Although the owner's manual calls for full power and 20 degrees of flaps during this maneuver, the pilot initially chose to set 30 degrees of flaps, and added only partial power. When the aircraft began to settle the pilot added full power and raised the flaps to 20 degrees. But, because the airspeed was decreasing and the stall warning horn was sounding, the pilot returned the flaps to the 30 degree position. In this configuration the aircraft, which was within 25 pounds of maximum gross weight, did not accelerate or climb. The pilot then concluded that he would not clear the trees at the end of the airstrip, and was forced to attempt a landing in an adjacent field. During this attempt, directional control was lost and the aircraft impacted the terrain.