Growing up I've always thought that you could never be too nice, but trust me, you can and if people can, they will walk all over you (the grand majority). I'm not saying to be hard faced, to be mean, or to be horrible but from experience, I would tell you to stick up for yourself and do things for you.
I've spent most of my teenage life doing things for other people, either to keep them happy or just because I was scared to say no when in reality, people would rather you said no if that is how you genuinely feel. Recently my anxiety has been taking over my body again but where as before I would put myself under more anxiety, stress and pressure by carrying on with my plans, or my friends plans I have now learnt to give myself time and to talk through my problems with people I trust.