How can I keep from falling off my diet? I can't seem to sustain it.

One thing to consider for life is that you don't have to diet. Diets are short-term, as you have made clear (i.e. not able to keep it). Perhaps instead of focusing on the perfect diet, instead focus on what a healthy lifelong strategy will be. You can maintain your physique regardless of the environment. If pizza is the only choice, just have one slice instead of half the pie. That is just an example. Ultimately, for this to last for you, you'll need to find compromise and control so that you are comfortable no matter the environment.