Your heels can be up as long as your weight isn't on your toes, which will tip you forward.
If you continue to ride nongaited horses, you want to try and open up your core. Your collapsed in at the trot (looks good at the walk). Like said above open up your chest (My trainer always told us to flash people with our headlights.... sorry little kids. LOL). When you open up, you'll roll back a bit on your seat bones and be able to use your core and leg to encourage this horse to soften.
The picture just looks tense, but not bad at all. You certainly have a decent EQ for usually riding gaited horses. Your leg isn't shoved out in front of you.
If your horse says no, you either asked the wrong question or asked the question wrong
And God took a handful of southerly wind, blew His breath over it and created the horse