Very few people diet just once, lose the weight, never have to diet again. Maintenance of the weight you do lose is a kind of diet, so anyone who diets even once stays on some kind of eating plan to keep it off. So, don't beat yourself up for trying to diet many times.
When you find the right balance, you'll know. It will be easier to turn down more food (or drink - if you're a college student, you're going to have occasions to drink alcohol!) when you are happy to know that it keeps your weight down. When you haven't hit your 'groove' yet, you'll be constantly adjusting and resisting. Keep at it and you will find what works for you - financially, emotionally, physically. Good luck!