My golf mental game is stopping me playing good golf. I really need to correct this so i can shoot good scores?

I'm currently playing high school golf. I practice every day and play two to three times a week. I'm currently shooting mid 80's but not consistently. I know i should be shooting low 80s consistently but my mental game is letting me down. The other day i was playing a 9 hole qualifier and was one under after 3 but still managed to shoot a 48. I really want to be able to break 40/80 on a regular basis but my mental game is preventing me from doing that. If i start well i usually finish with an awful score because i think wow this could be the time i break 40. Any advice?