I like the fact that the two previous commenters didn't tell you to start with something smaller. Most people do that. To me these guys have the right attitude, the "failed" attempts are no longer failed but learning experiences. Start with some high goal, and bring it down to where you are. This is what I've been saying to people for years.
Mark you just learned a valuable lesson -- that game development whether it be for a hobby or commercial purposes is work :-) Yep that dreaded work, we all want to be lazy and not do anything and have our games magically appear. The real world, unfortunately, doesn't work that way. As such we must work hard if we want to create something significant. I used to have two sayings... one is No Pain, No Game... This means you don't get a game done if you don't go through the painful process of finishing it. Secondly I found out you "had to go through the crap to get to the good stuff". Essentially what that means is the hard work, the boring busy work, the little details that add to the polish, the bug fixes which may not be fatal, but you know it's not working right, so you have to fix though -- basically anything you don't want to do the "crap" if you will, HAS TO BE DONE, if you want to finish it. If you ever want to be successful, you've got to realize this, accept it, and go for it whole hog.
I don't know if you remember Greks' puzzle challange, that was the 3d sokoban style puzzle that I made 90 levels to, while Blain designed and programmed the game. He got like one sale I think, and deemed it a failure. However, it was never "properly" finished, because we didn't wade through the crap to finish it. It needs a proper interface, it needs to be easier, the gui needs to look nicer, we need some extra goodies that will get people to try it, and there are a few bugs that need to be addressed. But Blain like any good person got tired of it. He worked on it so long that he didn't want to look at it anymore. And who can blame him, you work for a better part of a year, get it to playable, and then nobody is interested in it, because it doesn't look that hot.
Now I *KNOW* grek could actually make alot of money, if we only finished it... (Actually we might have finished it if I hadn't dropped the ball a little, and we still might yet).
What does this have to do with you? Point is you need to do the stuff on your game that you don't enjoy doing, and in fact you need to do that first. There was an article on "eat that frog" -- which translates to, do the thing you most hate because once it's out of the way, it's clear sailing. http://home.earthlink.net/~denmartin/etf.html -- if you're interested.
So... I think you have a good thing going with space fortress, don't let the things you DON'T want to do stop you from the things you do want to do, which is finish. Space fortress should be an incredible game when you're done. Good luck :-)