I have this problem!
Make sure your stirrups aren't too long. I rode forever in way too long stirrups. My legs improved immediately after I got them the right length. Generally, your stirrup length should be as long as your arm (arm pit to finger tips). But it isn't for some people. I have super short legs in proportion to my upper body, so I had to play with my stirrups for a while. Post a picture of you in the saddle and we'll see what we can tell you.
When you post, you aren't only going straight up and down. You're also going front and back. \ Like this little line. Diagonal. You post from your knees. Your knees are your pivot point; you bend them to move diagonally. Think of your posting movement as kind of like a hip thrust.
I don't know if this is your problem... But also try leaning a little forwards at the waist. Sit more on your crotch than your butt. Western riders sit very straight in the saddle. It's hard to lean to post properly without leaning a little forwards. (Don't take that and run with it though. Just a little forwards. Nothing that makes you feel uber unbalanced.)