It's definitely difficult to maintain that original motivation that arises when you first think up a really cool concept for a game...maybe even impossible. I've been using 3DGS on and off for somewhere around 7 years now, and countless projects have fallen by the wayside from lack of motivation over time.

I can think of a few things that help:
1) Set smaller, more realistic goals. For example, instead of telling yourself that you're going to create an entire RPG, make it your goal to just create an innovative battle system and then take your time and do that well.
2) Make sure you have a definite long-term goal. I think a lot of us non-professional game developers tell ourselves that we're going to make something really cool and then sell it and make some money. I think a better goal would be to develop your skillset and create a portfolio of your best projects. That way, if you decide to apply for a game design position later on, you can show them what you're capable of (which companies will ALWAYS want to see).