I've started writing the same thing in so many different ways, but here it is condensed into a single post and called what it really is: resistance.
That voice in your head that Steven Pressfield so accurately describes. It tells you you can't do something, that you're not good enough. It says, if only I had started earlier, if only I was already established, if only I had built up a body of work already. If only I'd done something with all those brilliant thoughts I had. If only I wasn't afraid. Ignore it. Push forward. Build momentum, continual forward motion.
I have so much I want to work on, to share, and it's time to stop letting resistance win. Writing is difficult. I've heard a number of writers say something along the lines of, "I greatly dislike writing, I love having written." The end result is worth it. It's the process that's difficult. As with life, it's also the process that makes it worthwhile. I read something the other day from Kobe Bryant, talking about his love of the process with results as a by-product. Across disciplines, the greatest performers and achievers have this in common. The love of the work, the process, the daily learning and improvement. Control what is in your power to influence. Pay no attention to what lies outside of this control. Trust the process. Trust yourself, work so that you can be satisfied with the work regardless of result. The work itself is the reward.
Just get it out. You don't get any better sitting there thinking about it. Things will change, you'll make new discoveries. You might look back and wonder what you were thinking. But that's who you were at the time. I heard Neil Strauss the other day talk about how bands will have an album ready and then keep tweaking it until it changes into something entirely different. Put out this version, reflect on where you are, and then go make the next thing.
Themes emerge out of the work, out of actually doing something. You won't find them any other way. It does no good just to think you're smart, like you've come to some grand conclusion. Does it stand up? Can you explain it, can you write it down? How do others respond to it? Can you teach it to someone? How do you know that you understand?
When I sit down to write, I feel deeply inadequate. I want to communicate all I know, I want the reader to feel everything I've put into the words. I have this idealized version that I can't live up to and end up doing nothing. I just want it all to come together. I want it to be easy, and end up making it so much more difficult. I'm shown just how little I understand what I thought I knew.
The way through is to find standards with which to measure yourself. Create and stick to deadlines. Constantly seek to discover how you work best. And most importantly actually write instead of just thinking about it.