Hi Zoe,
I agree, it is very difficult when you are writing about something that has been covered before by other authors. If you are trying to express an idea, it would be bad form to literally lift a sentence out of someone else's work and present it as your own. If you want to express an idea in your own words and then use a quotation from another writer in support of that idea then it is usual to express that as a quotation and cite the reference either in a footnote or and endnote. You may draw on other writers for inspiration, but it is your responsibility as a writer to come up with your own form of words to express that idea.
The important thing here is to remember that writing non-fiction is not an academic exercise. You should be attempting to achieve some goal. In these self-improvement type books, the goal is usually to get the reader to take some action. So, your contribution as writer is not to simply to express existing ideas, but to express them in such a way as encourage the reader to action - think of that as being your contribution to the exercise and focus on your unique way of communicating and you won't run into this trap.
Blessings,
David
