Few days ago I told my friend that I want to go on a camp, but now, when almost everything is settled I don't want to go. I don't want to dissapointed her, but I want to be fair with me what can I do?

Be honest, tell her how you are feeling about the situation. Remember, it is not about disappointing her it's about how you feel about the whole situation. See, sometimes pleasing others end up taking a huge mental toll on us. Talk to her and don't worry about disappointing her, because you might go, but since you are not enjoying it, you might end up saying or doing something that she even might get more disappointed. It's all about assessing your situation and doing what is right at the moment, you must also consider the long-term effects of your actions.