Best advice I ever got given was to forget the last ball and just play every ball on its merit.
I used to have a confidence issue and used to strive for perfection.... then I realised that the balls I edged in the nets and the bowlers were celebrating actually normally went for 4's in matches..... bowlers take no notice of soft hands in the nets lol
Seriously just enjoy your cricket mate and try not to have any fear of failure because we all do fail from time to time. I used to worry about getting out and looking good when I was around your age, it was the fact I felt that way that made me tentative and more likely to get out.
We all get nervous before we face up at the start of an innings but all I now do is as the bowler is running in to bowl in my head I'm thinking BALL, BALL, BALL to trigger myself to watch the ball as closely as possible..... I've found that it really helps me concentrate and the fact that I'm concentrating so hard on the ball means I've very little time to feel twitchy and worry about anything other than where the ball is pitching.... once I'm set I tend to stop thinkning "ball" without realising.... and I'm away.
You obviously have a good basic technique or you wouldnt be at the county so its about developing the mental side of the game too along with tweaking your technique which is what your coaches are paid to do.... become mentally strong and you are on your way to becoming a top player.