I've been doing a lot of reading and listening to pod casts recently, and a lot of successful developers stress making a bunch of games as one of the best paths towards success as a game developer/designer. I've repeatedly heard comments along the lines of "Don't try to make your dream game initially, just make ANYTHING."
I had recently been having some of the same issues you mentioned, but I decided to try using my hour lunch break at work to create a simple game each day. Admittedly, an hour is obviously not long, but if you are sufficiently experienced in some type of game tool (Flash for me, or Unity), it should be possible to make something during this time period. I've done it five times now, and the crazy thing is that I have five new game ideas done.
They have horrible graphics, are buggy, and often not incredibly fun to play, but the cool thing is that I have an idea now of the possible success of five new game mechanics, and I've lost less than 5 hours. One of the five seems like it could actually be a fun game, so I might spend more time to actually make it into something.
Yesterday, I happened to have found a link to a guy who made one mini-game or interactive program each day for 219 days! (see http://interactionartist.com/index.php?page=allgad) Imagine having 219 ideas and prototypes in less than a years time. Sure, a lot of them will be junk (many of his are less than stellar), but incredibly, many of them will quite possibly be fun and interesting ideas.
Creating something within a super-constrained time frame forces you to focus on the core mechanic of the game, the idea that makes it special. Sure, to have a commercially successful game you need polish and high production value, but "you can put makeup on a pig, but it doesn't change the fact that it's a pig" lol. How much better to spend a tiny amount of time on each concept, then you can focus on polishing the ideas that seem to actually have merit.
Doing so will increase your confidence ("Wow, I have X ideas functional in only Y hours?"), increase your portfolio, and increase your skill as a designer. Good luck!