But I always find a way to resolve these doubts and questions. As a wise friend told me recently when I expressed insecurity about my work, "plugging away is the only answer, since action has a way of resolving abstract problems like insecurity. Go and make it better." I did.
When I can't write, it's usually because I'm not clear about the topic I want to write about. In that case, I get clarity for myself by researching or contacting a client and asking questions or sketching out what I think the assignment is about and seeking confirmation. When you don't have a clear destination in mind, then it's little wonder you hesitate as the journey begins. It could also be that you don't have the passion or the capability to write about a topic. In that case, you should NOT write about the topic -- your lack of energy and/or knowledge will show on the page, making you and your client look bad.
So having clarity might be the first antidote to writer's block; having energy and ability is another; as is a certain work ethic (plugging away). But what about the mysterious muse and various psychological insecurities. Well, if you need to wait to be "inspired" to write, then you have an occupational problem. You're a professional, after all. You don't see plumbers or carpenters waiting for their muses. Writing is a practical activity, not that inspiration doesn't arrive on occasion, but work needs to be done whether the muse shows up or not.
If you have psychological insecurities such as low self-confidence or initiative-deadening perfectionism, then seek professional help. No piece of writing will ever be perfect, nor will any writer be perpetually self-confident. An ability to deal with the messiness of the writing process, not to mention the imperfections of human psychology (whether writers have more psychological insecurities than other people is a question for another post), is part of what writers need to work.
If you can't write a terrible, plodding first draft, as I have countless times, then you can't get to a solid fourth draft that satisfies the client. Embrace the process, or at least follow it knowing it's the only process available to you. Messes can be cleaned up, and this is what the writing process is largely about. Knowing this is the only real cure for writer's block.