Hi, well, you can look up stuff on here about how to say no to others and how to let your walls down. You can also buy books on how to say no. You sound like a people pleaser. Also, it sounds like you have some self esteem issues too. It's hard to open up to other people at times since there will always be a chance that person will reject you.

Not everyone will though. I'm introverted too. Maybe take baby steps in the meantime and just try saying hi to people and talk to the people in your class. Try to step out of your comfort zone. And start practicing saying no to people. It'll get easier with time. I used to be more of a doormat who rarely ever spoke up even when I should've, but I'm a lot more assertive with people now. It took me years to change though.