The pilot reported that he aborted the takeoff from the grass airstrip because the airplane was not gaining airspeed. He said that the airplane remained in ground effect after takeoff, but was not accelerating and he elected to land on the remaining runway. When the airplane touched down on the runway, it encountered a soft wet area and “slipped.” The airplane then veered off the runway, collided with an embankment and nosed over. The airplane sustained substantial damage to the wings, fuselage and empennage. The pilot reported no mechanical anomalies that would have precluded normal operation.