This happens to me often with my hobbies. I believe when I start a project I can see the finish line and my project is in an ideal state. When I get near the end I think I'm afraid I won't reach that ideal or I'm somehow afraid to reach it. I think it also has to do with learning a ton of stuff and that ideal I had at the beginning has changed and grown in scope so I'm stuck with finishing a lesser ideal or starting over.
For example, I wanted to write a book. Now, I'm not a seasoned writer. I learned so much while writing that by the time I was half way through, I was a better writer, and I looked at my older chapters with disappointment. I wanted to start over. But with more writing, will I always be looking back in disappointment?