Yes, once you've eaten that cheeseburger there's no reason to eat the brownies, fries and ice cream. But it's also a good idea to not START with the cheeseburger. Do you plan out your foods in advance? Are they delicious, yet lower calories? If you've got a planned menu for the day, it makes it MUCH easier to eat healthy. Eating healthy won't happen by chance or on it's own - it's got to be thought out ahead of time. It takes thought, effort and a continuous conscientious effort.
Losing weight is a doable thing. You must DECIDE to do it and then see it through. But by all means set yourself up for success - journal your foods - you bite it you write it - no exceptions. And then there's what I mentioned earlier - plan, plan and than plan some more. Failing to plan is planning to fail. Require more from yourself. Hold yourself up to a higher standard. Stop settling for foods that simply taste good. Let them taste good and be good for you. And remember - it's okay to tell yourself no. Discipline is a good things. Just because you want something doesn't mean you have to have it. You must remember what you want the most - to be slim, trim, fit, active and healthy. Every time you give in, you start the cycle again of having to start from the beginning so to speak. Work past the initial discomfort, because if you stick with this, it DOES get easier and easier. Allow these good habits to take hold.
You have the ability to lose the weight. You don't have to be fat if you don't want to be. Make the decision to do this, once and for all, no matter what and permanently.