I've felt all these same feelings before. The one project that I have finished was because my wife kept encouraging me to finish it and reiterating that she thought it could be very successful. Luckily for me, she was right. Find someone that can push you to finish things. Whether it be a spouse, a cofounder, or just a friend that will follow up with you and encourage you to finish what you've started when you both know it's worth pursuing.

Also, all the stuff everyone else said. Get customers first. Build the MVP not the final draft of the product, etc.

A couple other things that I do is to sit on an idea for at least a month before I start building. A lot of times I run out of energy for an idea before I even start (this helps filter out the ideas that I don't really want to work on). And I also really like to pick projects that I think I can create an MVP in less than 2 weeks. If it takes 6 months to build an MVP, then I am far more likely to run out of energy and question if this is really want I want to be working on.