Try to focus on using certain topics only - this will help you use the vocabulary you already know while practising what you want to practise - writing.

Let's say you want to write about nature, animals, travelling (anything you have extensive knowledge about) and then you use the vocabulary you already know. This will help you to not to be discouraged from writing. It doesn't make much sense to try to write about politics in czech when you don't know the vocabulary but if you use what you have and you will be patient with yourself it will pay off.

Please remember that just because your writing is not as good as Milan Kundera's doesn't mean you are not doing a great job.

Hope that helps.