Relapse is a very common phenomenon in addictive behaviours. It takes time to develop the familiarity and trust required to implement new methods of coping. In a pressure situation, you will learn to use your new tool rather than reverting to your old coping strategy. Until you have the strength and trust in yourself to cope effectively in the new way, you will often utilize the coping strategy which has worked best for you in the past, even if you have a strong desire to behave differently.
What is most important is that you appreciate that relapse is a part of the healing process and not a failure or sign of inability to change. Relapse is to be expected and welcomed, because it provides you with clear information about the situation at hand. The goal, when you experience relapse, is to use it as a tool for identifying stressors that are still challenging and for which you need to reinforce new, more healthful ways of coping.