My problem has never been allowing the mind to wander, it's always been chasing the damn thing down and getting to to do something constructive. Think outside the box they say, but what if you were born without one?
There's a certain knack to it, but it's taken me well into my late 30s to be able to do it "on demand" and even then it's difficult sometimes.
When I was younger, I used to try to force myself to concentrate but it was always counterproductive. At best I used to get bad butterflies in the stomach, and at worst migraines and aching muscles.
With the experience of age, I've learned not to be so strict with myself, and kind of potter around the periphery of what I intend to do, maybe just reading and analysing very superficially. When I get it right, and if I am not interrupted too much (that's something other people are responsible for but you can hide away from them or make yourself unavailable), I drift as if by magic into "the zone." Very often I find that an hour and a half have gone by without noticing it, and I get much more done that I thought possible.
The trick is to start off with something very easy, something that doesn't even feel like work, or like it requires any effort. If you can start with something very untaxing, the mind kind of wanders in to the main task...
Oh, and the doctor has been giving me all sorts of pills for many years now, too. :-)
There's a good one for anxiety-type disorders called Hydroxyzine. It's actually an antihistamine. I don't have to take it very often.