I second GeneSmith’s suggestion to ask readers for feedback. Be aware that this is something of an imposition and that you’re asking people to spend time and energy critiquing what is currently not great writing. If possible, offer to trade—find some other people with similar problems and offer to critique their writing. For fiction, you can do this on CritiqueCircle but I don’t know of an organised equivalent for non-fiction.
The other thing you can do is to iterate. When you write something, say to yourself that you are writing the first draft of X. Then go away and do something else, come back to your writing later, and ask how you can edit it to make it better. You already described problems like using too many long sentences. So edit your work to remove them. If possible, aim to edit the day after writing—it helps if you can sleep on it. If you have time constraints, at least go away and get a cup of coffee or something in order to separate writing time from editing time.
I second GeneSmith’s suggestion to ask readers for feedback. Be aware that this is something of an imposition and that you’re asking people to spend time and energy critiquing what is currently not great writing. If possible, offer to trade—find some other people with similar problems and offer to critique their writing. For fiction, you can do this on CritiqueCircle but I don’t know of an organised equivalent for non-fiction.
The other thing you can do is to iterate. When you write something, say to yourself that you are writing the first draft of X. Then go away and do something else, come back to your writing later, and ask how you can edit it to make it better. You already described problems like using too many long sentences. So edit your work to remove them. If possible, aim to edit the day after writing—it helps if you can sleep on it. If you have time constraints, at least go away and get a cup of coffee or something in order to separate writing time from editing time.