Hello~I am so sorry you are having these issues.
Sometimes depression can cause the brain fog and other symptoms you are having, as can anxiety. Are you stressed over school or any thing going on at home?
I would suggest that you take some vitamin supplements, mainly a high potency "B" complex, these are great for nerves, vitamin C and the herb Ginkgo, this is fantastic for brain fog. Also, try to eat more nutritious foods when you eat, make sure you add protein to each meal such as low fat meat, cheese, etc. Stop the cigarettes as well, they are not helping any health issues.
I hope you feel better soon.
Forgetfullness, a lack of concentration and foggy mind are all symptoms of depression and anxiety im afraid. There are a couple of things you can do though. First of all, ten minutes of mindfullness meditation a day has been shown to really help concentration and it drastically reduces anxiety symptoms. It sounds to me like youre stuck because you are finding it difficult to concentrate so you are finding it difficult to study which increases anxiety levels because you dont want to fall behind. You have to break the cycle by chilling out. Your concentration will come back on its own if you do this. try doing something that chills you out before you study. example: study after a bath. Another thing you can do which will really help immediately is exercise. I have depression and anxiety and i am in my final year at university. I can completely guarantee that if you hit the gym and really tire yourself out, then sleep for an hour, then get up and hit a diet coke and start studying you will find it easier to get in the zone. Smoking isnt good for anxiety but if you are only doing it occassionally its unlikely to be causing any real symptoms. - just watch your habit because it is a slippery slope when you arent feeling too good. You need to give your body some fuel. you say you dont like to eat but your brain needs energy to do stuff - also you should know that the brain typically goes to carohydrates and simple sugars first for energy before fats which is why a good exam tip is to eat a snickers bar right before you go in. Id recommend since you dont like eating so much finding something light that is pretty sweet like a banana before you study. The last thing is omega fish oils - you can get these in any health food store - they are just a vitamin but they contain the oils usually found in fish like omega 3 or omega 6 and taking them daily has been shown in studies to really help concentration. I hope this helps you. Also, you should know i used to be just like you and i promise the depression and anxiety thing gets better as you get older. its just something you get better and better at dealing with. one day you wake up and you realise that actually you dont feel so bad anymore. it takes a while but i promise it gets cured with time. hang in there and keep up the studying. remmeber just take your time and stick at it. im 25 and im only in my final year of degree now because i was homeless when i was 19. not gonna lie - it was difficult to keep up the studying while all that went on so i just went part time and took it slow - best thing i ever did. If things ever get on top of you and youre finding it difficult just always remember theres a third option and dont ever give up. i know how hard it is to keep doing daily stuff when youre feeling that way just remember youre an inspiration mate and itll be worth so much more when you finish those exams.