Actually, I felt it would be a good idea to write this out to explain a bit about a difficult matter in game development or any other art: Starting.
I'm not sure which is the harder difficulty: starting a project, or fixing all the problems along the way. Sure, there will be hundreds of minor snags later, but at least you've some logic to base your decisions upon. When you're staring at a blank slate, even if you've a good general idea of what you're going to do, you've no threads to grasp outside of those you yourself create.
Don't feel bad - it's not a matter of talent. Even if your brain is a font of unbridled creativity, even if your inner muse has given you her phone number (you lucky prick) you're still in trouble in that you'll have a hard time deciding which of the billions of cool ideas you should implement.
To these ends, my discovered solution has been this: begin to free write a design document. The goal is to try to assemble a good picture in your head how you're going to put this
It's important not to neglect the design document. You shouldn't even try to code until you've a relatively playable game down on paper. No, not every minor detail, that's just soul crushing, but at least have your major parts of the game down and know how they fit together. Without that document, you're setting yourself up to fail on many fronts: not only would you not know what you're getting yourself into, but you'll probably encounter some unforeseen incompatibility issue that would require you recode a significant amount of your project for it to work.
When you've started coding, you're still not done with the design document. If you've put off working on your game for a few days, this document is great for bringing yourself back up to speed. You'll be looking there every time you encounter a situation where you're thinking to yourself, "okay, I finished that part of the game, what next?" Leave notes to remind yourself what's left to be done. Every time you have a cool idea, tack it on to the end of the document (or a seperate document) in a cool idea pile. Your design document should be as much a work in progress as your game itself is.
Of course, every artist's motivations and methods are different. The Wikipedia entry on free writing helps to overcome "apathy and self-criticism," and perhaps the reason why it works for me is because I'm generally an apathetic, continually self-critical individual. Your hang ups are your own, and you may need to do a bit of soul searching and experimentation if this approach isn't right for you.