Did you have the problem before you started taking medication, or after? And if you've taken 15 medications, how long have you had the problem -- with that many meds it should be many years -- and how long did you stay on the meds? I ask because there is research showing that once certain people start taking antidepressants or benzos, the brain just doesn't work the same when they stop. These drugs can also affect certain people this way. But depression can also have this as a side effect.
It always seems that symptoms start suddenly, but often when you look back you can see it was coming. But if it did start suddenly, it could be a lot of things, including attention deficit, thyroid problems, other hormonal problems, blood sugar problems -- a host of possibilities. And it could be depression, I guess. As for the medications, going on and off so many meds and only staying on them for a very short period of time can leave your brain reeling for a while -- that's a lot of bouncing around for your neurotransmitters. But if none of them worked at all, then maybe it's not depression you're suffering from and need a better doctor.
I'm sorry to hear this. It will help to work with your doctor on the depression. Medication along with talk therapy really can help one overcome depression symptoms which things like poor concentration are often associated with.
I have another suggestion. There is a link to body movement and brain organization and thinking, even executive function. Exercise that includes things like light weight lifting, muscle work, swimming, running, aerobics, etc. all can have a positive impact on cognitive ability. Our nervous system can be malfunctioning a bit and exercise can improve things. I have a son with sensory symptoms--- there is a noticeable improvement when he is exercising and doing 'heavy work' (occupational therapy term for muscle work, deep pressure, lifting things, pushing things, etc.). My son's thinking skills, brain organization is better. Through research, there appears to be a proven link between physical activity and executive function and cognitive ability.
Food for thought. :))
I know it is hard when depressed though, so please work on proper treatment for the depression overall.