If you speak the language fluently, you are most of the way there. Modern business writing uses everyday language - the language of conversation and you are an expert.
Forget what you may have heard about about 'the formal rules of writing' (I'm quoting a few of my workshop participants).
There are rules of grammar, but you will be using most of those in conversations everyday anyway. Formal rules suggests 'commercial English'. That's the language of bureaucracy and I could show you evidence that it's been officially condemned for more than 90 years.
If you are not yet a confident writer, it's especially important to use simple sentences as building blocks. Keep to one thought per sentence - that way you won't run into grammatical problems. Join compatible sentences together with words like and, so and but.
If English was a disaster for you, maybe you lost confidence early on. Maybe it was the poetry that put you off. Maybe it was boring.
Maybe you were more interested in science or maths and you believe the myth that you are either a words person or a science person.
You can be both. One of my colleagues has an honours degree in physics. He is also one of New Zealand's most prolific writers - of novels, text books, articles and even a screenplay.