This is more a confession than a tip, because I’m as guilty as everyone else in failing to complete many of my personal projects. I’ve failed so many times that I’ve learned to actually live with it.
Let’s be fair, not all of our ideas are great and sometimes we realize it in the middle of the work. If I lose my interest in the project after a couple of days of working on it, I know I won’t do my best job and I won’t be proud of it at the end of the day anyway.
On the other hand, you may never know unless you really start working on it.
So, my advice is to stop daydreaming and get your hands dirty as soon as you can. If you lose interest too soon, it may be OK to drop it and look for something else that you may actually fall in love with and never quit.