The question title is "How to pick...", but you seem to be asking "How to complete personal projects?" I think there are (at least) two things going on:
Something that seems like a good idea at the time may turn out to be less important to you after you've thought about it for a while.
The form of your work may make it more difficult to come back to. Sometimes, when you're feeling inspired, you have the sense that you can see the entire project and how all the parts fit together, and you want to write it down before it gets away from you. When you come back to it later, the code you've written may not be so clear, or may not be so well designed as you thought it was at the time. Or maybe you've had some additional ideas on the topic, but they don't really fit in so well with the original concept.
In the first case, not finishing might be the right decision. You've had an idea, it was interesting, and now you're moving on.
In the second case, I think that feeling of seeing everything is your brain playing a trick on on you. You've got an idea and it seems great, and your brain races around filling in the missing pieces to make the idea seem more coherent. One way to deal with that is to slow down and use a better design process. Instead of jumping right into code, write the idea down in prose. Create a high level design, and then start fleshing out the details. If the insight you have relates to implementation, then write that down too, but don't try to code it all up at this stage. This way, you've captured the idea(s) on paper so they won't get away, but you haven't implemented anything, so the design is still pretty flexible. At this point you can come back to it later if you want, and if it still seems like a good idea, improve the design and eventually start implementing in a methodical way. Keep a log of your progress so you can always tell what you've done and what you still need to do.
The last thing is that you have to actually want to finish the project. If you don't want to finish it, then it's just another chore on your to-do list that piles up and makes you feel swamped by a ton of work that you don't actually need to do.