As George Morris says "You are not ontop of your horse, you are wrapped around your horse"
You have to rethink your position while on your horse and you have to understand where your solidity comes from, when in your saddle.
First and foremost, your heels are very important in the whole grand scheme of things - they must beable to be your anchors. When we allow our heels to anchor us in our tack, we find that base of security, we find that solidity and we find that the rest of our body can be functional as well.
So, in order for our heels to be our anchors, you must allow our bodies natural weight flow, occur. That weight must beable to flow from our heads, down into our seat, and down into our heels. The moment we grip or pinch with our knees - we block that weight flow from occuring, so we loose that base of security in our lower leg.
So back to what GM says - start imagining yourself as being wrapped around your horse. Open up your knee's, get them off your tack, and allow that weight to flow down into your heels naturally, then wrap yourself around your horses girth.
Make sure your legs are under you, you are balanced over your feet. Open your knees and allow that weight to flow from head, into heels. Don't block it from occuring. Allow your heels to do their job :)