Being heavy on the forehand does not matter much. It is still less heavy than the forehand of a giraffe.
When a centaur is making his front part upright as shown in your figure, his center of mass will shift a little backwards and upwards compared to that of a horse. He might be more stable than a horse because the weight of the front part will be directly on the forelegs while the neck and face of a horse are leaning forward.
