Here's a video that might help! So much of Natasha Althoff's videos and content are valuable for so much more than just dressage, even if that's what she focuses on.
My suggestion to you is this: 1) follow the advice in that video about not having stirrups too short and not gripping with your knees, 2) try trotting without stirrups to develop the correct muscles and a longer leg, and 3) make sure to STRETCH before you ride (stretching advice in a video below.)
It's about your back and core, not your legs. 'Heels down' has a few different purposes. One is as an indicator of where you are in core strength, and if you're able to use your core strength to keep yourself upright while you flow in motion with the horse. If you aren't sitting up straight and engaging your abs and core (yes, that so rudely includes your groin muscles) then where are your legs? Are they relaxed, but prepared to give cues to your horse?
The second purpose of 'heels down' is as a way for you to allow downward impact energy to flow down through your stirrups, which is a part of moving with the horse and actually 'riding' rather than having every muscle tense up (which I affectionately refer to as 'planking' on the horse.) We work to have everything be very fluid, and our 'core' is what holds us up straight.
(In anything involving fast movement and jumping, heels down has yet another purpose that I think we all know and can appreciate. Those who don't do it end up dusting themselves off.)
Keeping your heals down, in stirrups, during the sitting trot is one of the hardest things you can do, so here's another video that might be helpful:
One of the things she says is to absorb the impact of the trot 'bounce' down through your ankles and into your heels. You can see her do it. And really, that's the secret. Stirrups should have very little to do with keeping you in the saddle unless your horse is having a mental moment. And hey, it happens, particularly if you and your horse are learning together. Don't worry about it too much unless whatever you're doing is causing the incorrect behavior in your horse. Having discipline drives you to be better, having fun is what keeps you smiling in the saddle.
Hope that helps! =)