Thank you. I am a native English speaker with a lot of doubts and low self-confidence, when it comes to writing :(
Can you advise?
Yes, you need to speak English to people, and to read lots of good English material. I had guessed that, since you are from the Netherlands, your native language, or at least best language, would be Dutch. It's very difficult to give general advice to a native English speaker who cannot choose between "How do I ..." and "How can I ..." I'd need to know the reason why you haven't mastered that yet. If it's mainly due to interference from Dutch, then you mainly just need to practice English, as I've mentioned above.