I find the easiest way to help students learn to post is by having them ride in the half seat for awhile. See if your instructor will let you practice your half seat for some time at the beginning of lessons, particularly at the trot. Riding in your half seat (when done correctly) really helps get your legs and heels down in the right position, what you need is to build the muscle memory in your legs to stay like that while posting.
There are lots of things to remember to do while posting, which is why hovering in a half seat while trotting is a great way to really learn the rhythm and build the muscle memory without having too many things to think about, then it becomes easier to find and follow the beat while posting. You want your calves on the horse and your knees soft, not pinching, you want to stand up and move your hips forward and then let yourself come back down in a controlled manner, not just falling back like a lump. This takes a lot of muscle. You can practice using these muscles by standing with your back against a wall and crouching down with your knees bent and your legs a bit in front of you, using your legs to hold you up, not your back. Just a minute of that should leave your thighs pretty tired so start slow!
There's a lot to remember, heels down, thumbs up, don't pinch with your knees, look where your going, all while posting is a lotto get! So don't stress too much, riding is not a simple thing to get :) Sounds like you're doing very well!