Ayush, Before anything else, please seek help if you are feeling hopeless. This answer is not being written as someone who is a Student expert here but is being written as a Psychologist. It is not a sin or a taboo to feel hopeless or defeated. It is an emotion just like happiness or joy. But this is a negative emotion and that cannot be held within too long. The good thing is you do not want to lose or fail and that's a start. Everyone who has a boasti g career now has had to face failures behind the screens. They might have shed many tears behind the sheets. But what sets them apart is their grut and determination through which they have overcome their shortcomings. If it is just the fear of failure and nothing else, it is fine. But if it is a deep rooted cause, please seek a Psychologist or speak up to someone whom you trust. English isnt just the begininng and end of the world. How many successful people speak in English? And how many good many people good in English have an amazing life? However, to improve your English try these books:

Word Power Made Easy (by Norman Lewis)
This is the go-to book to improve your vocabulary. It is not a book to be read but a book to work with daily to enrich your vocabulary.

High School Grammar and Composition( by Wren and Martin)
It is a very good book for people who want to improve their grammar skills. It helps to understand and conceptualise different aspects of grammar and it is a must have book for anyone with poor grasp on verbal ability or who wants to make up with the basics of english.

Read books from different genres
The ultimate key to improve your verbal ability is to read extensively and to read books from different genres. This ensures that when the time for reading comprehension comes you are well acquainted with different writing styles and topics. Reading extensively also improves your reading speeding over a period of  time. There are no shortcuts here.

Try to put your fears, emotions on paper or in short story format. Ask someone to correct the gramatical mistakes. Watch English movies. Learn a new word every day. If you are afraid of being humiliated when you talk to people, talk to plants or animals in English. Talk to non living objects. But just talk and that is the only way to improve English. Feeling hopeless and crying heart out made no one successful.

One thing must be cleared that you are not late at all . The minimum age to appear for SBI PO exam is 21 years old. And you are only 20 years. So you have time . Don't feel like that you are lost, your friends are going over you. Don't go for competition. There are students who qualify SBI PO in first time. But there are more students who can't qualify in the very first attempt. But they don't lose hope. They stuck to it until they reach their goal. So when you have time, you must prepare for the examination rather than thinking all these.
First of all go through the entire syllabus of SBI PO. There are many study materials available on internet.   You can take help of video tutorials available free in YouTube. Read newspaper daily thoroughly for General Knowledge and current affairs. Once done with syllabus you can move on to solving mock test, previous year exam papers. Then try to figure out your weak areas and work on them.