No, I haven't got ADD, but sometimes I feel like that. I have so many ideas that I'd like to explore on keyboard that it's hard to settle down and choose one. Just about the time I get started on a historical, do all the research, figure out the characters, and outline the plot, I'll be skimming through my files and think, "Oh, there's that thriller I did two years ago. Maybe I should dust that off and do some editing; it was pretty good."
I don't think it's altogether a bad thing, and I'm sure people much smarter than I am, Leonardo, Edison, and Julie Garwood, just to name a few, have interests that are scattered over a wide area. In this world of who-knows-what-will-sell, it's good to try different genres or sub-genres until you find your voice and what you really love to write about, even if it makes you feel like you're all over the place.
What's required, though, is the ability to focus enough to give a project the chance to come to fruition. You have to say to yourself, "I'll make a note of that other thing I'd like to do, but right now I have to finish this historical." Sometimes it doesn't work, and I think when that happens it's because that project isn't quite ready to be born. If I spend a week trying to make the historical come to life and it doesn't, I give up and go on to something else for a while.
Which is what makes it a good thing to have so many ideas. I'm never not writing.