i had a similar problem at your age, but I've learned a few techniques to avoid feeling exploited. I still have to work on it regularly, but I've made a lot of progress using these tips:
-realize that you have the right to say "no." nice people sometimes have problems saying "no" when they want to, or when they should. practice saying "no." a simple, direct, "no." start with small things. "do you want more salad?" "no." gradually build up to saying "no" to more important things. and this might sound silly, but it can be helpful: go someplace where no one will hear, and practice saying "no" in a loud, firm voice. try _yelling_ no. No is very powerful.
-when you say "no." you don't always have to explain why or justify your decision. in some cases, you owe an explanation ... but in other cases, you can simply say "no." and when they ask why, you have the right to say, "i simply don't want to. I have the right to say no."
- think in terms of "boundaries." like a fence or wall around your life. you have the right to establish boundaries for your emotions, your body and your life. you have the right to allow people in your boundaries (or not let them in). boundaries can take a lot of work, especially if you're not accustomed to do it and if people are used to violating your boundaries. those fences around your life might need a lot of repair and re-building. but it's very important.
-be patient with yourself, and with others. sometimes, I've been angry at people who I thought were using me, when I really was more angry with myself for saying yes when I really meant no. Stuff like this is a constant learning process. So pat yourself on the back when you do something good or try something new, even when it doesn't work out as you hoped.
i recommend you devote some time to a website that has a lot of excellent videos. it's aimed at Christians, but not overbearing about the religious stuff. I'm not religious, but I check in on their site regularly ... it's very practical stuff.
Here's a video on verbal boundaries, such as saying "no": link
And here's a longer video on boundaries in general: link
They've also written an excellent book on boundaries, which lots of libraries have: link
I hope all this helps. Good luck.