Katty--
You might look into the amino acids. They are the natural building blocks of all proteins, but there is a subset that cross the blood-brain barrier quickly and can focus the mind a lot.
Everyone is different so I hesitate to even suggest. Lysine is one of the "inhibitory" types (as opposed to stimulatory). I would think that the stimulatory ones would hype you up & not encourage focus.
They are available without prescription and are reasonably priced. You can PM me and I can hook you up with some basic materials about them if you want.
I've had the opposite problem--never turning off. Just the other day I realized that I watched a whole half hour news show and didn't pay attention to it. That is extremely rare for me. In some ways it's a good thing for me, although I don't want to get too drifty!!
Lora