It's actually weird, I've known about my ADHD for a long time now, and up until just recently I thought "brain chatter" was normal for everyone to be honest.

I think i may have gotten better at stopping brain chatter and racing thoughts by pushing myself more and more every time my mind drifts off. When my mind starts to wander from whatever I'm doing I have to push myself... I used to have to do it constantly, and even other people would yell at me and stuff to get me to focus, but now that I've been doing it for so long I hardly ever have to push myself to concentrate.

I hope this helps, but I know it can be frustrating. Just try it and be patient about it. Remember not to be too hard on yourself when your mind wanders.