i think you should practice byyourself more . when you play byyourself (and i mean practicing) you may not feel that presure and missing shots won't affect you that much. Try potting some balls by yourself and after a period of training try it with another player. Practice is everything , and...
The thing is you have to work harder and never give up. Exercise , practice every time you have the ocasion , play against better opposition and be patient!