Smoking and going downhill.. Need encouragement?

lately in the past few months I've been smoking, eating a lot more than usual, and stopped going to the gym like I used to. I've gained a lot of weigt also. I'd like to quit all of this and go to the gym and get back on track again but I feel like a failure. I'm onlyy 16, does anyone have any advice? Have any of you gone throug this? I feel alone and disgusted with myself.. :(