This old saying was never more relevant than it is to the process of overcoming an addiction. Although it is great to start the process of quitting with the attitude that you will stick to your goals 100%, the experience of relapse is so common that it is actually sometimes recognized as part of the cycle of change, for example, in the stages of change model.
And approaches such as motivational interviewing will help you uncover your reasons for falling off the wagon, which can actually strengthen your chances of sucess in the future. For example, if it was cravings that cause you to relapse, your therapist can help you to learn strategies for dealing with cravings when they happen next time.
Unfortunately, 12 step programs are not always so understanding, so if you are finding your relapses are making it difficult to participate fully and honestly in a 12 step group, you might consider switching to motivational interviewing. Contact you local treatment center to find out where it is offered.