There is one very simple piece of advice that I have found helps me with this:
finish.
It doesn't matter how small the finish is, but whatever it is, at least get it to a point where you have something finished. If you are writing a book, and you want to switch, finish the outline. If you start a chapter, finish it. If you start an app, have at least a hello world page up.
If you don't feel like working on it now, and a new idea pops up, feel free to start the new idea after you finish. Practice finishing just as much as you have practiced starting.
finish.
It doesn't matter how small the finish is, but whatever it is, at least get it to a point where you have something finished. If you are writing a book, and you want to switch, finish the outline. If you start a chapter, finish it. If you start an app, have at least a hello world page up.
If you don't feel like working on it now, and a new idea pops up, feel free to start the new idea after you finish. Practice finishing just as much as you have practiced starting.