What Causes Lack Of Concentration And Memory?

Hi, I am from india(bangalore). I think I have brain fog symptoms from quite long time. I have problem in concentrating and focus. n my short term memory is too bad and confused most of the time. I feel fatigued most of the time. I got my b12 and thyroid tests done. I had MRI scan. but every thing is normal. I have dust allergy and can t tolerate cold as I easily affected by sinus. Can you give me any advice on this.