It sounds like you give up too easily. I think I have the same problem, so I'm really just writing this because I want to change myself, and it helps me to write shit down.
If you end up never finishing anything, ask yourself why you really stopped the last project you had?
Was it because you were genuinely bored? Or was it that you just didn't know what to write next? Or is it that you just enjoy planning an idea, but not actually doing it?
Starting new things and planning things are easy, because you don't have any constraints and any view of all the details coming ahead.
A lot of people don't seem to realize that, but you're much better off sticking to a project until you have even just a good part of it you can call done.
Even if the original idea wasn't that great, at least you'll have learned something and gotten better (and proven to yourself than you can get shit done).
Everytime you start something new, you might get excited at the fresh possibilities. But the truth is if you haven't finished your last idea, you're just proving one more time that the way you do things leads to fucking nothing getting done.
So don't, every time you want to start a new project, instead go back, pick a thing, and see if you can improve it until at least part of it is done.
Even if the result is shit, at least it's done, you can learn from the parts that suck, and maybe just maybe next time it won't be shit anymore.