I think at least everyone has a little bit of trouble saying no to people. We get in the habit of always saying "yes" when someone needs a favor. We want to be known as the one that other people can count on.
But sometimes, we get so swept up in wanting to be liked and the go-to person for maybe our boss or even a friend, that we forget about ourselves. It's important to take the time for us to focus on what we need to accomplish. Whether this is for our classes or even taking time to focus on our well being.
Don't overload yourself by saying "yes" to every little thing that is asked of you. Prioritize the things you have to get done and make sure you set aside enough time to do that. If you are able to do this, then you can decide what else you can take on.
It can be hard to say "no" to your boss, teachers, or even your friends. But it's also a necessity to look after yourself and not take on more than you can handle. Take a day for yourself.
Take the day to slow down and only do the things that you enjoy doing. Make a cup of tea or coffee, whichever you prefer. Read that book you've been telling yourself you're going to sit down and read but has been sitting on your shelf for three months.
Sometimes it's okay to say "yes" to people because you do want to see reliable. That's important in different aspects, especially when you want good references for an internship or new job. But help yourself by not adding on so many responsibilites to where you are so stressed out you don't want to do anything.