I started out posting with my feet, had just the same problems you describe, over time shifted to posting off my thighs, and now am returning to bringing my feet back into the equation. All the natural evolution of learning how to post properly.
If you don't have your feet take a part in the action of posting, then you will end up gripping with your knees or calves, heels coming up and losing your stirrups. Also, many people who have their legs too far out in front of them and have that "waterskiing" kind of position is because for one reason or another, their leg being so far forward means that they post off their knee and this mean that the horse has to work hard to lift them and they will be perpetually behind the motion of the trot.
You will be striving for an aligned position with you heel/ankle lining up with your hipsocket, lining up with your shoulder socket and your ear.
This way, you carry YOURSELF, like a puppet that goes lightly up and down by the accordian action of hip/knee/ankle. The weight is spread over all parts, but ultimately goes with a focus downward, out through the heel, which is pointed at the horse's back legs.
Yes, you do kind of move your pelvis a bit forward and backward along with up/down, but you want to be in a position that if your horse magically disappeared out from under you, you would land with a soft , bent knee stance and keep your balance.
Cheers to you on your new venture and take heart, it takes time to learn how to post. Take video now, because in a year when you wathc it, you'll laugh til you pee!