Dear Ajitu,
I'm sorry for being harsh with my words earlier. You still didn't tell us your age! There is no relevance of the advice to your age, but I just wanted to know what phase of life you are in so that seniors here can give you better advice.
We all have gone through or are going through the feeling which you have mentioned in your post, so what is happening to you is not new. However, you have to ward off this uncertainty and focus on your goal. All the advice mentioned by dear members is good, and you have to strive to follow this advice; only then can you truly achieve your potential. I know this is easier said than done, but you have to do it.
There will be distractions, so learn to focus.
Maintain fitness, exercise, jog, play your favorite sport.
Avoid useless, wandering, aimless friends who just call you to chit-chat, waste time, money, and energy.
Read books. If you don't like reading books, read articles, magazines, but good ones (Competition Success, India Today), or relevant to your field.
If you have absolutely nothing to do, listen to soothing music. Relax.
Never keep your mind idle.
Set result-oriented deadlines for the tasks you take up, write them down on paper, and paste them on your room walls. Then strike off whatever you completed; this will give you good satisfaction when you strike off a completed task.
Manage your time, set alarms, reminders, etc. Keep your schedule personal and private; don't drumbeat to your friends and colleagues about your schedules.
Respect your time, work, and efforts. Don't allow distractions to deter you from your goals. Don't let any temptations distract you - pretty girls, cricket matches, partying, festivals, etc.
Don't forget to have fun, socialize, make friends, go partying, but within limits of time and value.
Remember one thing: in this world, a self-dependent, rich, earning person is respected no matter what he/she does, rather than a depressed, poor, dependent person. If you don't buckle up, the world will put you down and judge every action.
So, my friend, you are not alone in this world, but you are truly alone in your head and heart. It's your battle, and only you can fight it.