Me too. And, to say that all these things that you struggle with, I would say at least 50% of the folks on here struggle with them , too. Me, ya, you betcha!
I had a lesson a while back where my teacher did this kind of interesting thing:
Sit the horse (horse stands quietly) and sit up tall but not forced. let your arms dangle down. now, roll the arm from the shoulder so that the palm now faces outward. Change nothing except the roll outward. Feel the slight pull across the front of your chest/shoulder? Now, holding your shoulders in that position, bring your lower arm up and around into "holding the reins" position. YOur shoulder should still be rolled back and your upper arm pressed lightly into your ribcage. Think of a million dollar bill in the back of your armpit. If you open your arm off your ribcage, you lose the money.
This is just to put you in a good position. It has to be somewhat flexible, like Anebel said "functional equitation" , but if you start to recognize that feel, you can go back to it whenever you notice your elbow rolling outward. think of it as "home" , and go there often.