While conducting joy flights a misunderstanding made the pilot believe that a commercial requirement necessitated landing in the 25 direction, which had a 2 degree downhill slope. The first approach for landing was high, and a go-around was initiated from about 100 feet above ground level. The pilot reported that during the overshoot the aircraft performance was poor, failing to climb satisfactorily and passed close to the tops of a group of trees. The next approach was still high and at a speed slightly faster than the published approach speed for the prevailing conditions. The aircraft floated for a distance after flaring, and touched down late. Being reluctant to attempt another go-around, the pilot tried to stop the aircraft before the end of the airstrip, but it overran and continued down a steep slope, coming to rest in a ditch. Witness reports indicate that although the wind at the time of the occurrence was described as a variable gusty crosswind, a certain amount of downwind component had previously been noticed.