Hey! Don't be so hard on yourself. Posting is not easy and getting it is hard. I found when I first started that I was trying too hard. I wasn't called Sarah Sit Two as a nickname for nothing. The trainer was always yelling at me to sit two because I was on the wrong diagonal. One thing I found that did help me was to let the horse trot for a couple of steps before you start posting. You sort of feel like you are being lifted out of the saddle then and that is when you post. When you do that you get the feel of the horse. Also, if you learn to ride english first it makes you are better western rider. You are better balanced in the saddle.
Keep going, one day you will look back and wonder why you found it so hard.