As a GM with this problem sometimes I can explain why I am like this...definately not due to ADD though.
For me, it comes down to two things: One) There are so many fricken cool games that are made today and I come up with cool ideas for many of them and want to try them that it's hard to pinpoint what I want to run down to one game, and Two) When I propose my ideas to my friends who could be players I wait till I get a reaction from them.
What normally happens in my group is this: If it's not d20, I tend to get only one or two people willing to give it a shot (that sucks to me because I think there is a whole lot more to gaming than just D&D) so those ideas get shot down. If it is d20, I get about half willing to try. The other half simply state: "If it's not D&D, I don't want to try." (I really dislike this kind of thinking because it is sooooooo damn limiting, but that's my opinion).
So, I am stuck with D&D if I want a decent sized group, in which I am at my best.
Then I can come up with a lot of ideas for D&D, and my problem is that I do have a lot of ideas and it is hard to limit them to one.
Another problem for me is that when I usually start my games, I tell them that I require backgrounds, character goals, interactive characters with the world and my goal in starting the game is to run it so the players invest in the world, the characters get invested into the world (and the players) and that without backgrounds it's just hard for me to get their characters to interact with the world. I'm a Give and Take GM...the players give me something to take, and I give back to them. What pisses me off is that most of these people I tend to game with simply argue with me about backgrounds, about alignments, about everything, even before I begin.
So, I tend to give up and try something different.
This bothers me even more when I ask them what kind of game they want to play before we even begin talking about the world and campaign, and all of them say, "I want to play in a game that will give me that deep roleplaying feel where I can immerse my character with the world. I don't want to just fight all the time." I think
Great, its about time I found some people who can think outside the hack and slash box. but all they really want to do is play hack and slash.
That's why I tend to start and stop so often, because I simply don't get what I want from the players. I've tried getting new players, finding new groups. For some reason, whenever I make a decision on what I want, and what house rules I use, or what guidelines I come up with for making characters, I always get the players who just argue with me.
Sometimes GMing these games can be really frustrating.