To fully concentrate in your performance it is important that you are "in the flow" ! The flow is a state of mind in which you are totally concentrated and focused and your performance almost just goes on by itself.
To get into this state of mind you can use various techniques, one important thing is that you are in the "here and now" in the present.
To do this you need to quiet your thoughts and your mind and almost stop thinking. Stop thinking also means stop talking to yourself!! Did you ever notice that you have an inner voice that sometimes is not helping you, on the contrary it is making you even more tense??
Many people get very nervous because they start telling themselves things that are not useful so you can follow these steps:
1) Listen to what you tell to yourself and slow down your inner voice,
2) Tell yourself only thing that will put you in a useful emotional state and if you find it tough at the beginning tell your inner voice to "shut up" and let you concentrate on what you are doing
3) Take some time to relax, breath in and visualize you show
4) Smile and laugh if you can to expel tension
5) to keep your concentration focus on one thing at a time and imagine that all the surroundings disappear and that there is only you and your horse left and just concentrate on the two of you!!