I once had a young lady tell me, "I would like to be an author but I have about 3 or 4 ideas for books in my head and I don't know which one to write".
"I do the same thing", I said, "I think most authors do. Just pick one".
I have since come to understand that when you are writing a book and have writers block, that can be a great time to start one of those other books. It keeps ideas flowing. Writers block is just a lack of ideas. When you start writing another book you may find you have a lot of ideas for the second one that will help kick start some ideas for the first book you are writing. keep that brain working! Happy writing.