Mr Spice,
What has been said before covers nearly all the main points. If nothing else it was a very good lesson in some respects. As an Instructor I teach all my students that if the speed the height or the flight path isn't stable its a no brainer Go Around. If it was as severe a porpoise as you described you probably got away without damaging the a/c partly down to softer (grass) surface. Although I appreciate I wasn't there I am suprised your instructor didn't intervien.
The key is to learn from other peoples mistakes, without being morbid I always read the accident reports and of all the landing accidents that result in a broken nose gear, propellor or engine the last line of the report always say's I quote:
"In a full and frank report submitted by the pilot he commented that he believes the accident could have been avoided if he had made an earlier decision to Go Around".
Safe landings
