I saw those screen shots on OnGameDev and they looked absolutly stunning. I think youve got the common problem of trying to do too much in your game. Take the time to simplify it and you'll find you can get it completed.

Change the structure of the game to being level based. Each level you need to fly over the idlands and shoot down some enemies.

Forget about landing on the islands, make a single landing point at the end of the level where you get your next 'mission' and have the chance to upgrade/buy new weapons etc.

Believe me - learning to simplify your game to a basic set of requirements is the best way to get to make complete games.