Being an introvert or a shy person is very tough to survive in this rough world. Plus when you don't know how to say no, the game is already over for people like us. 
When someone asks me for a favour, and even if am not capable of doing that , but I will say "oh yes, sure" and then I will regret the exact moment. They would ask " would you mind..if I ? Is there any problem?"
"Haha, No , Sure! There is no problem, please." Huhhhh... Then I take a deep breath, calming and telling myself "okay,chill it's okay, you are a nice girl, let's help them."
Sometimes (no, actually most of the times) when people  come to know that you are not good at saying No, they start taking advantage of you. 
1. I spent my entire night in making an assignment that my professor really appreciated me for my rough work . The next day one of my classmates came to me and straightly asked if I can share my work with her. I was in shock for a couple of seconds but then I smiled and handed over my file to her "sure,take it."  Chalo koi niiiii
2. Last night my roommate was blending something for her lab work ( may be charcoal) with the help of a huge stone or brick may be. I was about to sleep after a hectic day. And then she started this. She looked at me and asked if there is any problem ? I replied " no no, continue I will sleep later." Only I knew how badly it was hurting my head. 
3. Even my roommates alarm causes a lots of disturbance in the morning. Her first alarm goes off by 5:30am. And after full 3-4 snooze with a gap of half an hour (30 mins) She wakes up. Because of her I sacrifice my sleep. I have only one single alarm 07:00am and I wake up straight with a single bell. Thinking that I am no one to disturb others. Everyone loves to sleep and there sleep should not be bothered. Why can't they think like me ? I am really very afraid of asking her to turn her alarm off. Also Her phone calls also disturb me when I study. So I will go to the reading room instead of asking her to talk slowly or go outside.
There are so many times when I feel pity for myself. I just need to learn to tell people that I  feel disturbed by their acts. And I should be telling them that they need to stop the things that cause problems for me. But when I try to do that, my throat gets choked and I forget what I was about to say . The only word that comes out of my mouth is-  " Yes, sure" !

Thanks for reading this!