I have trouble with the goal post exercise.

Make sure you aren't pushing into pain and instability. Stop your range of motion just short of where it starts pinching and feeling very unstable.

Your muscles need to get more familiar with the position JUST SHORT of the problem area.

The only way to get more stable and to build range is to practice it carefully and mindfully.

I can't externally rotate my shoulders enough to get into the goal post arm position.

Don't fret. Keep working on that external rotation at least twice a day! Keep practicing those tough positions. The consistent practice WILL pay off. 💪

Also doing some stretches like those found in this video will be helpful. https://www.youtube.com/watch?v=_-oHVOrZEWw