Plan ahead for your lunches. This is a critical step in maintaining a healthy diet. Don't leave it to chance. If you do, you'll be much more susceptible to fall into old habits and reach for something unhealthy. The next time you go to the supermarket, make a list and buy specific ingredients for your lunchtime meals. Make sure you prepare them ahead of time. It is important that they are convenient, nutritious, and delicious!
Give yourself a break! An occasional indulgence in a high-calorie treat won't negate all of your hard work. It's ok to diverge and "cheat" every now and then. Just make it the exception and not the rule! Do not give that treat so much power. Instead of focusing on guilt or feeling bad about yourself, just get back on track by trying to fit a little extra exercise in over the next few days.