Keep fighting. The more you resist, the stronger you become. I've said it so many times it's getting cliche, but your willpower is like a muscle. The more you use it, and the more it is challenged and put to the test, the more powerful it gets.
And don't just fight harder. Fight smarter too. Learn the patterns that lead to failure. Avoid behaviors and activities that result in relapse.
Be patient, and don't give up on yourself. When you fall, get right back up and keep moving. It took me five years to start seeing noticeable progress, and another year to finally break free. But it's worth it in the end. Hell, if it had taken 20 years with ten times as much difficulty, it still would have been worth it.
Fight on, and never give up, even when you give in. You will make it in the end if you stick with it.