What is the fastest and best way

Your main problem is distraction.
你主要的问题是分心了。

You were using the laptop for 5-8 hours, and you tell me you are unable to concentrate. You can effortlessly use the laptop for long hours because it is engaging and interesting to you, right? This means you have the capability of doing one thing for 8 hours.
你能玩5到8小时的电脑，到你却跟我说你不能集中注意力。电脑对你来说很吸引人并且很有趣，因此你能够玩那么长的时间，是吗？这就意味着你有能够在一件事情上集中8小时的注意力。

It is just that, academics might not sound interesting to you if you have the option to browse internet. Your sleep cycle has clearly disrupted due to inappropriate usage of laptop.
只是如果你可以选择去上网，那么学校的东西对你来说可能不是那么有趣。由于使用电脑不当，你的睡眠周期很混乱。

What is the fastest and best way to improve my memory, cognitive skills and span of concentra 如何又快又有效提高自己的记忆力，认知能力和专注力？

Here is what you need to do:
给你一些建议:

1. Set your sleep right: Sleep at 10 PM and wake up at 6 AM. The most important thing is to get a good 8 hour sleep. If initially you don't feel sleepy at 10 PM, just get to bed. Reading a technical book in the bed will definitely make you drowsy. If that doesn't work, just lay in your bed, keep doing this continuously and you will get the habit of sleeping right.
   养成正确的睡眠习惯:晚上十点睡觉，早上六点起床。八小时的高质量睡眠是很重要的。刚开始时如果你十点钟还不觉得困，那么你只要躺在床上就行。这时读一本技术类书籍肯定会令你十分困倦。如果没用的话，只要一直躺在床上就可以了。你可以建立一个正确的睡眠习惯。

2. Know your priorities: Get a notepad (not digital) and a pen. Before sleeping write what are you going to tomorrow.
   拿一个记事本(非数字式的)和一支笔。在睡觉前写下你明天要做的事。
   Remember, First things first.
   记住，重要的事优先做。

When deciding whether you want to use Facebook or study discrete Mathematics, make the wise choice. Sit in a separate room or go to a library without your laptop, and if possible leave your phone at home (or at least turn off the internet on the phone). Set a goal, only get up for washroom/eating. Keep practicing engaging problems, that is the only way you can improve your analytical skills.
当你决定是使用facebook还是学习离散数学时,记得做出正确的决定。找一个单独的房间或者是去图书馆，但是不要带电脑，如果有可能的话，连手机也不要带(或者至少把网络关了)。给自己设个目标，起来洗澡或是吃早餐。不断地联系有趣的问题，那是提高你分析能力的一种方法。

Deactivate Facebook for 10 days.
10天之内不要用facebook。

3. Nothing rises suddenly in the world, not even the sun.
   冰冻三尺非一日之寒。
   

   What is the fastest and best way to improve my memory, cognitive skills and span of concentra 如何又快又有效提高自己的记忆力，认知能力和专注力？
   I can't stress on how important this is, keep going, no matter how many times you fail, but learn from failure and improve on. Start small, meditate for 1 minute, do it no matter how many thoughts intrude your brain, keep going, because being thoughtless is not really the aim of meditation. It is the pinnacle of practicing meditation. Meditate daily. Even if it is for a minute in the first week.
   我不能强调这是多么的重要，坚持下去，不管失败了多少次，从错误中学习和提高。从小事开始，每天冥想一分钟，不管有多少想法在扰乱你的思绪，坚持下去，因为什么都不想不是冥想的目标，是冥想的顶点。每天冥想。就算在开始的第一周只有一分钟。

You know, I was going to learn how to swim. I saw videos on youtube, wikihows and other related stuff on how to swim, when I went to the pool I was disappointed, it took me 3 months to learn properly, so you need to be at it, improvement is not an overnight change.
我打算学习游泳。于是我在youtube wikihows和其他相关的网站上看视频来学习游泳，但当我到游泳池时我感到很失望，它花费了我三个月的时间来学习，因此你需要去实践它，提高不是一夜之间就能达到的。

If you follow all these steps, you are definitely going to get rid of all your problems, but it is very important to do everything as written in the answer.
如果你按照谢谢建议来做，你肯定能改正你的问题，回答中写的东西对于做任何事情都是很重要的。