Small, small, small games. Once you've finished enough of those you'll have the motivation for a big, big, BIG game. Or at least a moderately sized one. You should never force yourself to finish something you're not enjoying. I have dozens of prototypes sitting in a big Dropbox folder, a few of which nearly became games. Sometimes though, you just lose interest and that's ok. I often work on multiple projects but rarely does more than one become something, sometimes none at all.
One thing that helps me a lot is to not plan and just work from your head as much as you can. Also try and do menus and background systems and all of the boring stuff ASAP because I'll often have a nearly finished game stuck without a main menu for weeks because I hate making menus so much.