If you have a chat with the player, then stating the obvious is often the simplest and easiest thing to say. Just beware of painting yourself into a corner by saying something like, "If you do that again, I will caution/send you off". It's better to be vague but obvious, such as, "If you do that again, I MIGHT have to caution/send you off". I wouldn't worry about keeping a tally of mistakes in game as there may be a subconscious attempt to re-dress those mistakes by making incorrect decisions.
Far better to sit down a few hours after the game or even the next day and reflect on your performance. Consider when you felt low in confidence, ask yourself why you felt like that? If it was because you got a dispute over a decision, is there anything you could have done to either make the right decision (if on reflection you got it wrong) or to be more convincing in making that decision (the way you signalled, the viewing angle of the incident, your proximity to play when it occurred, etc.). If you find you have common factors in each of the decisions where your confidence was sapped, then decide how you could become more effective in those decisions.
On the other hand, it might be that you had a troublesome player who you didn't deal with effectively. If that's the case, then consider using the stepped approach by having a quiet word, then an obvious one, then a major chat (bring the captain in to support you at this time and only talk to the captain, not the player, on this one), before moving to caution and then dismissal. Persistent offenders can make your life more difficult if you ignore them or don't take action. Deal with them effectively and other players may see this and fall into line without you having to take any action directly with them.
Final point and this is for Colt ... if a challenge is reckless, then it is a caution, not a chat. Teams don't like referees who don't take action when they should.