Sounds like you are working for a good chef. One that understands it takes time to build up and gain experience before you are a star. Garnish is one of the hardest stations because it can get so hectic with making all the stuff to go with the protein. It is usually a "starting" station on the hot line but damn, it can get crazy.
Try not to make the same mistake twice. You WILL make mistakes, it is just part of learning, but if you constantly make the same mistakes over and over there is a problem. Iterate each day on your learning...fix the problems and work that into how you prep and how you cook. Nothing drives a chef crazier than making the same mistake over and over again.
Keep a notebook...dishes you do, plating, recipes, etc. Write notes to yourself on how to work better.
When I was young I would show up for work 1-2 hours early and work off the clock (not usually recommended by some people around here, but it really helped me get all my stuff done when I was new and a lot slower). I did this to make sure I was set up for service...nothing in a kitchen is (in my opinion) worse than being behind all day. If you are ready and twiddling your thumbs at 5pm waiting for the first tickets, that is GOOD. Once I got into a routine, learned the fastest way to prep, knew my pars, etc, I could come in at a "regular" time and still get it done. Showing up early is a good way to show your chef you are serious about what you are doing and committed to being the best. Again, it is totally your call, but if you get shift pay or salary it especially might help you to show up a little earlier. I don't know how it works in Europe. You might have some people here tell you to never work for no pay, but again, it is your call. I did it, and I know a LOT of cooks and chefs who did it, especially when they were younger and just starting out, in order to be set up and learn the ropes.
One last thing...don't let mistakes sink you. Fix your mistakes, obviously, but don't let a mistake effect the rest of your service. FIX the problem ASAP, and move on. I've seen lots of cooks have a bad start to a night, get flustered, and spiral out of control for the whole service. You have to mentally be able to move on, not take it personally, and come back with more fire to do it right.
Good luck...welcome to the big leagues!