Ok, as finding a way to improve my ability to focus and learn is an important topic for me, I'll add another answer. Biased, as I am a medecine student, critically looking at both medecine and the alternatives (more critical about the alternatives, based on experience with some stuff).
Sleep is important, no discussion about that. I have experience with insomnia, so you should trust me on this
About food: yes, good quality nutrition is very important. But I would like to add a bit to some of the points mentioned:
-ketogenic diet, which is very popular these days, can be very useful in the short term. In the long run, consult your doctor, please. You should definitely have your liver and renal functions measured before such a huge and longterm change.
-the brain eats glucose. That's it. It doesn't matter where do you take it from, what does your body make it of. It does matter to your other organs. Today, medicine completely agrees refined sugars and added sugars in general should be avoided, or the intake should at least be reduced. It also agrees that the old tendency to leave out fats and eat more carbohydrates was wrong, misguided, and harmful. Avoid "light" products like plague.
-One of the problems tied to eating crap is fatigue. If you don't eat enough protein (I am leaving aside the issue of the proper source of protein, that is not the important thing here, and there are more opinions about it, I am for good quality meat in the appropriate quantity, not too much and not too little), you are likely to be tired. You are therefore not gonna learn much, logically.
-Yes, drink a lot of water. Headaches are not good for learning either. Again, medicine agrees.
-Vitamine D is still being underestimated, but mainstream medicine is finally taking note of it. It can help against season based depression, and therefore improve your learning capabilities, if you tend to suffer from it. Many people do, perhaps the vitamine D could help you. It cannot hurt. Vitamine C can help against tiredness sometimes as well.
-Organic isn't always the best. Sometimes your local farmer can give you the same or better quality food, just without the expensive badge (yes, the "organic" or "bio" certification is a huge business, not a pure paradise island in the cursed world of the supermarkets). Sometimes the officially organic product isn't that different from the rest either. And technically speaking, most stuff we eat has been genetically modified. Selective breeding. We have been genetically modified too, as our ancestors were always trying to find a good quality mate, not a random one. Usually, no extreme is good, choose your own position on the whole scale.
Simply, use your brain while deciding how to feed it. We are different. What suits one person, may not suit another. One of my best friends and I have lost over 12 kg over the same period of time. The same height, the same weight at the beginning. But one of the most important things for me was starting to eat much more meat and diary. For her, it was important to leave most stuff like that out and focus on plant based alternatives. That is just a real life example why you should not blindly follow advice, even from someone with great results. It's your body. It's your brain. Get to know them both.
I wholeheartedly agree: Be critical. Make informed choices. Don't blindly follow any advice, learn. As House MD says: "Everybody lies."
As I am already seriously responding to this, I would like to add a few more things I have tried, or I know others who have tried them.
-Omega fatty acids. Quite popular these days, I am trying them out too. Cannot hurt, and it looks like they really can be helpful. After all, whole generations grew up hating fish oil. Their mothers knew, why they were forcing it in their kids
-Cognitive drugs: I wouldn't ever risk that. Some medicine students do and it supposedly works nicely (which makes sense). And I am pretty sure they are not getting those from their doctor (the insurance companies would eat them alive). But there are no long term studies concerning the possible secondary and long term effects.
Please, don't risk that, any medicine playing with your brain can be dangerous. -Ginko, Lecithin, and so on. I tried some stuff like that, I didn't notice any effect, some people use it. Either it helps them or it is a good placebo. It cannot hurt and that is important.
-Treating psychiatric problems. There are many untreated depressions and anxieties and other such stuff in the population, as there is still the stupid stigma. If you think you may have the symptoms, please, consult a good specialist. Either they'll tell you "you are within the norm. Come again, if it worsens" and you can be just as calm as if you went to your dentist for a check up and heard the same stuff (and your supposed symptomes may even improve, based on such a reassurance). Or, you can get help (which doesn't have to be pharmacological in many cases. A good psychiatrist is definitely NOT trying to prescribe stuff to everyone they meet. The 21st century has arrived even to this field. And there are as well non-medical specialists, a good therapist is more helpful than a doctor in a huge spectrum of problems.). In such a case, your performance in all areas of your life is likely to improve.
I have found methods like the pomodoro useful, but there are many people better at
time management than I am, I am sure their posts concerning this subject are worth reading at least twice.
One more thing: the support from people around you. Either they are supportive, or they don't need to know about some stuff at all (almost noone needs to know about my Italian experiments ). It helps not to listen to things like "give it up, you are not gonna succeed at both this and that, you are not good enough" regularily. A loving partner, who believes in you and your abilities, can do miracles. Family that reassures you, that is a huge gift. Friends reminding you how awesome you are and helping you get up after a failure. Confidence is highly important. During the last month, I have been struggling with my studies A LOT. Both medicine and languages. Lack of confidence has been a big part of it (I've heard a lot of "give up" talks lately). Let's fight that!