I used to have the same problem. My trainer would always yell at me to put my leg behind the girth where the horse could feel it. I don't do it as much anymore. It could be the saddle. Certain saddles make it near impossible to keep your leg underneath of you.
is it (Slightly) behind the girth at the two point?
I do a lot of two point and then when i go back to posting i try and keep my legs in the same position. When I first started doing this, i would almost start posting in the two point and then lean back to ensure & smooth transition from the two point and that my legs stayed in place. (because I was told i 'jerk back' to the posting position and loose my leg so a smoother transition between the two stopped that).
Also we have a dressage ring w/ a mirror in it. I used to ignore the mirrors because it was too much of a distraction, but now that I'm really focused on my leg position, it was amazing to see and very helpful. I could totally tell when i had a nice, stretched leg, totally weighed, heels down...... and when I didn't.
Next time you ride, drop your stirrups and ride for a few strides. Allow your leg to fall where it is naturally comfortable. Then stop and look down. Is the stirrup hanging just by your foot or ankle, or is it hanging in front of your foot? Is it hanging too far below your foot? It it falls too low your stirrups are too long. If your stirrup falls in front of your foot when your leg is comfortably beneath you, then you need to switch saddles or add a riser pad (though changing saddles would be the ideal solution). If this is not the case, then it might be a muscle or balance issue on your part, or it might be the seat of the saddle. Do stirrupless riding to build strength and balance, and try different saddles to see if your seat improves.
I had the issue too. I knew better than to have my leg as far forward as it was, and I couldn't figure out why it was moving forward. Eventually, I tried repositioning my saddle by moving it back a few inches and my leg improved. You could always give that a shot
I have this issue. I've ALWAYS had this issue. Or the opposite problem of it being too far back, but that only rarely.
Try riding standing in your stirrups. Not in two-point but actually standing, with your thigh vertical and your upper body straight and tall. Can't do it, I bet. Then try moving your lower leg back. You'll feel like it's way out behind you, but you'll be able to do it now! Now drop your knee. The only way you can do this is to bend it more because your ankle will already be dropped as far as it will go. This is your "seatbelt" position and you will go NOWHERE with your leg in that position. No matter what your horse does you're not going anywhere. To keep your knees down, take the biggest dog you can imagine and pretend it's sitting on your knees. You physically cannot lift your knees. No matter what you do they will NOT come up OR forward.
Viola, no more "PUT THOSE HEELS DOWN!" from your instructor. Because they physically cannot come up when your "dog" is there and strong.
It's VERY hard, and very frustrating, but it works!