1. Fear of not living up to expectations.
This affects published writers - will my next book be as good? - and the unpublished - I've talked about my book for so long, it'll never be as good as everyone expects. I think you have to ask yourself what the alternative is to not writing a book. What would happen if you walked away from it? Sometimes looking at the alternative makes you less afraid of failure, whether it's because actually, the alternative isn't so bad or because you realise that all you really really want to do is write.
2. Fear of the internal editor
A lot of us have an internal editor who sits on our shoulder and tells us what we're doing is worthless rubbish. Sometimes the voice of the internal editor may be that of a former teacher or a parent. Try to visualise your internal editor. Then tell them to go away. Explain that you're busy writing and part of the process of writing is to write rubbish because we can edit it later. Writers often call the first draft the dirty draft. Accepting that your first (second, third...) draft is going to be rubbish is part of the writing process.
3. Fear of Rejection
This is a pretty obvious one. No one likes getting rejected. No one looks for it. And sometimes it hurts so much that we'd rather not expose ourselves to the risk that we might get rejected. So we don't write. That means our dreams of being a writer are safe. But...if we don't write, we're not writers.
Edison said in an interview: "after we had conducted thousands of experiments on a certain project without solving the problem, one of my associates, after we had conducted the crowning experiment and it had proved a failure, expressed discouragement and disgust over our having failed to find out anything. I cheerily assured him that we had learned something. For we had learned for a certainty that the thing couldn't be done that way, and that we could have to try some other way."
Think of writing as statistics. There are X number of publishers out there. At least one of them will publish you, but it may not be obvious which one. So you have to send out to lots of them before you get the right one - but, each time you get rejected, you're closer to finding the right one for you.
4. Fear of Success
A friend of mine confessed that she doesn't send her work out because she feels she couldn't cope if it was accepted - they'd demand another book for starters. So you're back to the fear of demands being made of you, and the fear of expectations. In fact, you don't have to write another book, you may well be offered a one book deal. But the one thing you can guarantee is that if you don't write it and send it out, you'll be offered nothing at all.
5. Wrong time, wrong place
I made lots of attempts at writing a novel and short stories in my 20s. None of them worked. In hindsight, I don't think I had anything that I really wanted to say. Later, when I started writing again some fifteen years later, the words flowed. This time I had things to say and was prepared to learn how to say them.
I've had times since when I've been completely unproductive as a writer. They're always associated with other things going on in my life such as the time I took on a job I loathed or my father's sudden illness and death. There's nothing you can do about this except trust that time will bring back the desire to write. Be kind to yourself.