Trying to break a bad habit is simultaneously the simplest and hardest thing in the world. The problem is the brain is a creature of habit and can work against you when you are trying to make healthy changes. One way to reduce your stress when you are trying to establish new healthy habits is to remember that you don't lack willpower when you have a craving - you have a healthy brain that is trying to get you to stick to your (albeit, unhealthy) routine.
Mindfulness is one tool you can use to help change how you respond to cravings. I love this TED talk by Judson Brewer on his research on mindfulness and habits. Another good resource is Jeffrey Schwartz' book, You Are Not Your Brain.
Understanding why your brain has cravings, along with support and a plan, can help you calm your cravings and start living healthier and feeling better than you could have ever imagined.