When going from enlisted to officer, what was the hardest habit or mindset for you to change once you were commissioned?

Being overly close with junior enlisted or NCOs. Although you must be respectful as a commissioned officer, you are the leader of your personnel, not their friend.

Being in charge with limited experience. In garrisons the NCO's usually run the show. Where you make your money is in the field. So I suggest you learn about the Troop Leading Procedures, OPORDs, and tactics. Get yourself a TACSOP and learn everything about it until it becomes second nature. What formation does my tank platoon use? What do I do if artillery is hitting us? How do I go on the offensive? What do I do in the defense? These should be second nature to you.

Basically to let the NCOs exercise their plans rather than mine. Usually LTs don't have the knowledge to operate as an NCO but when you you can't act like one. You just have to sit back and let them do their duties as an NCO.