If you've been playing 22 years and have only completed a handful of games I think your ADD probably extends beyond just games. I might change from one game to the next in a session, but I always eventually complete my games (except in the rare case when it's a truly irredeemable title).
I'm not big into "story" games. Aside from games like Mass Effect or Deus Ex, where a good part of the story and interaction with it are in your control.
If a game goes too long without letting you get immersed in the actual game mechanics (or doesn't have much depth in its mechanics period), I'll quickly lose interest and likely have to force myself to continue.