I'm been writing for over 40 years and I still read books on writing. I highly recommend books by Bob Bly. He has written more than 70 books plus dozens of ebooks - most of them about direct mail, advertising and copywriter. Go to www.bly.com
. I have several of his books and I read them again from to time whenever I get stuck.
That said, remember, writers write for different reasons ...
Novelists write to entertain.
News reporters write to inform.
Advertising/direct mail copywriters write to persuade and sell.
(Don't listen to any advice that suggests that copywriters need to be creative and clever. Creativity doesn't sell. Persuasion sells.)
One idea you might want to try is to put away your laptop and take out a voice recorder. Then, instead of writing your copy, organize your ideas and talk about them into the recorder. Then transcribe into text and editing.
What this helps you do is get all of your ideas down on paper first so you don't get bogged down tweaking every sentence.
There's a great line from one of the old time direct mail gurus who once said, "It's not how you write, it's what you write that is important." In order words, the content of your argument is more important that the way you phrase your sentences.