I won't drop my stirrups in case I lose balanced, but in order to force myself to sit, I have to consciously relax my legs and pick my feet up off the stirrups just a little bit so I cannot brace against them. I found my seat pretty quick. Just remember to [sit down] and keep your shoulders back. You have to sit up and you will fall into it just fine. AND [BREATHE]. Please breathe, lol.
Over thinking the canter and my seat, and trying to move my pelvis with the horse, well let's say I looked very snaky in the saddle lol. Don't over think, don't worry about how to move. Just sit down and relax. And practice. A lot. I've spent entire hour long lessons cantering circles around my instructor just to relax and sit and get used to just sitting and breathing.
Think of it not as a failure but as a success in how not to do it.
Don't look in a horses mouth for a gift.