You need to work on being more in control of your own feelings and to not worry so much about how others will think of you - don't give people that much control over how you feel. It is very hard to do and takes a lot of practice and perhaps some talk therapy with someone you trust, e.g. a therapist.
Truly, this is a psychological problem in assertiveness that you can study and improve over time.
The interpersonal skills part of this is trivial, once you work out the deeper issue of being in control of your feelings. Then you will be able to say, "no, thanks" or "no, I really can't" much more confidently, instead of worrying about appeasing everyone around you.
With more training and experience, you might find that you can even anticipate ahead of time when certain people around you are going to ask you of something for which you want to say, "no", and you'll already know how to answer them with assertiveness.
As an example, if your friend asks,
"Nitu, are you coming out with us tonight?"
you can say,
"No, but thanks for offering."
If they ask again, you can say,
"No, thanks, I'm not really up for it. Have fun, though, and thanks for the invite."
Remember: the interpersonal skills part of this, i.e. how to communicate your way of saying "no" is not nearly as important as first understanding why you let others have so much control over how you feel, and then, regaining that control, with practice and reflection. Focus on the core issues.