The way I explain it to my beginner students is this: When you are posting, think about standing up out of a chair. (You will not actually be standing, this is just an analogy to give you the feeling.) If you are in a chair and are preparing to stand, you pull your feet back underneath you to support your body as you rise. If you try to stand up out of a chair with your legs stuck way out in front of you, you can't get up without a struggle. There is no support there. So bring your lower leg back underneath you as you would to stand up, and it will put it in the correct position for posting. A quick way to self-check is to glance down at your knee. If you can see your toes sticking out in front of your knee, your foot is too far forward. You should not be able to see your toes if your leg is where it belongs. Like anything else, it just takes practice practice practice, until your body gains muscle memory for those things. You are doing fine for someone just starting out.
One other thing to think about. If you are really cramming your weight down into your heels, you could be shoving the leg out in front of you. You want to think about sinking your weight down into the heel, not forcing it down. Again, its just going to take practice. Hang in there.
"Keep a leg on each side and your mind in the middle"