I wouldn't take much notice of words like panicky, flustered, overbearing etc They are just outward appearances. The problem seems to be much more fundamental - you haven't been taught how to lead - which is no great problem, very, very few people can do it without being taught.
The thing with learning to lead is that you can do a lot of it from observation, so you don't need to start arranging expeditions to the highlands to get practice. If you have access to any cadet organisations, try and observe leadership exercises with whoever is instructing and ask yourself questions, what should the leader be doing now? what should they not be doing? how have they given out tasks in the team? what are their priorities? are they the right priorities? is everyone included? is the leader thinking of individual skills? has the leader considered the special limitations/dangers of this task? is the leader encouraging? is it obvious who the leader is? has the leader got too involved in the task detail? etc etc
Also, if you are in a work or sporting environment, look at leaders, critique them, see what works well and what doesn't work. Remember, at least at OASC you have a team of reasonably bright individuals who are usually doing their best.
You are right, it is an unusual letter to get from OASC and you should take heart from the fact that they have bothered to treat you differently, they obviously did see something they valued in your interview and aptitude tests. That means that you are the right sort of person, so again, don't worry about the descriptive phrases, and concentrate on learning the techniques of how to lead a team.