Hello there,
21years is not late, it is the ideal age for starting your SBI PO preparation. Do not stick with the memories of your past. You were lazy but that doesn't mean you cannot change yourself. See, from your class 12 incident you can understand that you have the capability to do something. You thought that you would fail but instead you scored 60% which is better. So you see that if you try, you can do it, because you have that capability. Get up and start studying from now. If you work hard, you will definitely get good results.

Some suggestions about your preparation:
You should know your syllabus, and focus on your studies. Believe in yourself. Don't lose your confidence, be confident that you can do it. Keep aside the negative thoughts and concentrate on your studies. Complete your syllabus keeping enough time in hand for revision. Solve the previous year question papers. Give regular mock tests. You can only achieve your goal if you are confident enough.

For more information about SBI PO, check the link given below:
https://competition.careers360.com/exams/sbi-po

Hope this answer helps you.
Good luck!!