How can I stop being so dim-witted?

I find myself being so dumb a lot of the time. My mind wanders very easily and I get distracted just as easily. I can find it hard to focus and to concentrate. Some people round me find this a tad bizarre as I am quite an intelligent person (IQ 128) and am very good at maths. But my memory sucks and I find it annoying as I constantly find myself asking questions about stuff I have just been taught (i.e. at a place of work) and it's usually simple stuff. I feel my mind is lazy, I had a very lazy period of my life between 2005-6 where I didn't work, nor did I have the desire to. I feel this caused my mind to switch off and not fully come back on again. I'm 24 going on 25 and am worried about any possible future impact. I also lack a lot of common sense. I was diagnosed and treated for depression last year and am OK now, but I don't really know what to do. I used to take a regular dose of caffeine to help me concentrate (energy drinks, coffee, caffeine pills) but I got hooked and am not doing this anymore. I want to train to be an accountant but if I get there I am worried I'll make a lot of mistakes and it will cause trouble. I've had this happen in a previous job where a lapse of concentration caused me to blow out the entire factories' power during a simple PAT test of a small power supply (I'd wired it up incorrectly!) I also suffer from a lack of self-confidence. While out of work, as I am now, I try to keep my mind limber by doing puzzles, studying different things, etc. but this doesn't seem to be working. Any tips?