Lots of excellent advice here, but I'd just like to add (and please correct me, anyone that disagrees - I am not the biggest expert here):
braking on surfaces such as gravel = be wary of using the front brake quite as much/hard. In these conditions, and in the wet, the back brake is your friend, especially with 'progressive' braking.
That's how I do it, anyway, and it seems to work for me. My first spill was pretty similar to yours (too far left into the crap piled at the side of the road trying to avoid something).
By the way, I'm a little surprised at your instructor's advice. When I was learning, if cars built up behind me, I learned to pull over periodically to let them pass* - are you sure that's not what he meant?
It's not worth getting a hernia over, or risking some dick rearending you or attempting a dangerous overtake.
*(Incidentally, by that I mean pull over and stop, not wobble to the left hand side of the lane and slow down.)
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