I don't know much about psychology, but I can tell you this and I hope it helps: your lack of concentration is a symptom of depression, but it really depends on how long its lasted and if their are other symptoms like fatigue, insomnia, hypersomnia, or if you have been down in the dumps for at least 2 weeks. But keep in mind this may be A.D.H.D or A.D.D (if you haven't been diagnosed with these already). Well, I'm not a psychologist, but I hope this helps.