A few things you may want to try is while shooting for score keep yourself psyched up, keep a jamming song in your mind that really gets you fired up and play the lyrics in your mind while hammering Xs and 5s. This will keep you pumped it plays with keeping your mind in a positive state of mind and it helps to keep you thinking about your score. When you do let one slip deep just let it go the arrows shot and the best thing you can do is smile at the person next to you, reload and hammer an X. here again your dwelling on a positive act (smiling) and the guy next to you will more then likely throw an arrow – why he’s standing there aiming and thinking about why on earth were you happy with the miss thus not focusing on his shot that is only his mistake not yours. Here again the power of the conscious mind, while the subconscious takes care of firing the bow.
Also if you can shoot your game until you either accomplish your 300, and or your new goal of achieving say 50Xs and building your way up 60 after that. It may take you a few extra rounds but in time your conscious mind gets used to the new level in success and scores and in turn it will be done without having to shoot the extra arrows.
Just a few things that may help you achieve to your goals, Good Luck