My personality is just too nice ? Help ?

Hello, so I am a 16 year old girl who's a senior in high school. Since I was a kid, I was a VERY nice person like I would do anything for my friends and the people in my cl. It was just this year that I developed myself a bit and I would say no sometimes or not do whatever they want. Let me give you a few examples on what I'd do: I would print 30 papers for 6 of my friends because they all don't have printers and I would do it for them, and if I ran out of ink or something I would beg my father to take me to the nearest stationery so I can print it for them, I'd always give them rides if we go out on a weekend even if they lived a million miles away. I'd always give them my homework to copy and sometimes I'd write their homework for them because they were busy doing something else. I always give them some of my food or water even if I wanted it and it became something so ordinary that they would open my bag and drink and eat without even asking me. I never argue with them, and I always listen to EVERYTHING they talk to me about no matter how boring or stupid it is. They make fun of me sometimes and they annoy me and I don't say anything, I just sit there and smile with them, even though I know they're joking but sometimes it hurts me. We all annoy each other a lot, it's part of our friendship but they annoy me and tease me a bit more. They'd only call me if they wanted to study something or do schoolwork but that does NOT apply to all of them. We're all 11 girls and out of 10 probably 5 or 6 do that, the rest I'm really good friends with and we're actually best friends. I love all of them and I know they love me back, it's true they probably use me a lot but then we're still amazing friends outside school, we do everything together but I'm still new to them so I don't really know what's going on. This year I've learned to argue and say no and I don't go with what they want me to do but out of friendship I go out of my way sometimes just for them. I can't live without them but at the same time, the way they treat me doesn't feel right. Sometimes, when I think about it I feel depressed and when they ask me what's wrong, I lie to them and tell them I'm fine because there's no way I'd tell them you make me feel that way. I need your opinion, on what is my personality REALLY like ? Should I change it more or keep it the way it is ? Do you consider what's going on improper ? What should I do ? Any advice would be great, I appreciate all of your answers. Thanks.