Firstly, you should ask your instructor as he was there!
If this was your first landing on grass and with only 13 hours experience, your instructor should first have demonstrated how to do it and then taught you how to do it.
"I set up the base turn well but came in too fast.. (80 knots).."
So what did you do to control the speed?
Was the flap setting correct? Full flap for landing!
What were you doing with the throttle? If the speed is high reduce power, possibly closing the throttle completely.
To land on rough grass you must be at the correct speed which might be 5 knots below the speed you use on a concrete surface (see Flight Manual), if not, go arround.
You must have a stable approach if you are to stand any chance of making a successful landing.
What was the instructor doing during all of this?