Don't worry, every gymnast will go through stages of wanting to quit and being frustrated, but being frustrated is not a good reason to quit. You will know when you are ready to move on from gymnastics because it will be for positive reasons not negative ones, you will feel content with the desicion and be ready for something new. Until you feel that way then you dont really want to quit, you're just frustrated.
There are lots of things you can do to help these situations. Why not try keeping a training diary. Write in it (or type in it, keep it on the compuetr) every night after training. Write about how you felt the training went, the things that you were frustrated and unhappy with, the things you want to improve on, things you did well, funny things your coach or team mates said, your goals, things you are excitied about and enjoying and so on. Some days you may only want to write a line or two and other days you may want to write heaps. This can really help, writing down your problems just helps you come to terms with them and work out just what is going on, it is also a chance to write about all the wonderful things that happened.
When you come home from gym frustrated sit down and think about the things you did well. You never get through a whole training and not do something well. You will probabaly be pleasently surprised. Design a plan of action for the things that upset you. Look back on what you have achieved in the last year and see how far you have come.
When you are at the gym and you have mental blocks slow down and try the five step procedure.
Step 1 - Head check, are you concentrating on what you should be doing? Are you thinking about getting the skill right or are you thinking "oh no I'm going to stuff it up?". Is your mind fully on the job? Refocus your self and have a few more tries.
Step 2 - If step one fails try visualisation. Close your eyes and picture yourself doing the skill perfectly. Try to imagine your body doing the skill and what it should feel like. Imagine yourself correcting what your coach is telling you to correct and then have a few more tries.
Step 3 - If steps 1 and 2 fail. Go back to basics. Go back to the drills you learned to do the skill in the first place. It may seem like a step backards but it is not. Even if you have been doing the skills for years gop back to the basic drills and it will help to remind your body what it should be doing. Then try again.
Step 4 - Seek help, of course your coach is there to help you all the time. But it can help to seek help from someone different. A different coach, an experienced gymnast or even a fellow team mate who may have had a similar problem with a similar skill. Sometimes it helps to get a new set of eyes to look at your skill, they may be able to find the words to get you where you want to be,
Step 5 - Walk away, and try again next time. Some days it just isnt working and the best thing you can do is just move onto something totally different.