I was told to try and stretch my legs like I was trying to skim the ground with my heel
I agree will Hayley. Great exercise and beneficial for rider fitness.
Position exercise - bridge reins in one hand, place free hand half way up the crest of the neck, eyes up and ahead, two point contact out of the saddle. W/t/c/g
With a correct position come correct heels. It's all part of a package.
Staying up in 2 point in walk, trot and canter. Making sure you are balanced, try stretch down through your legs. That's how I learnt and it really helps!