I thought i would answer your question/write a post about this as i am sure you are not alone with these thoughts and maybe the changes in daily routines.
The first thing i can say is that these cahnges in your lifestyle are OK. Life will always change and your routines and habits will have to change or adapt, and you cant let that change stop you from doing what you want.
When you study or work your body needs alot of fuel as your mind is always working and using so much energy. I dont know whether you were studying before hand, or if you have had a long break or just had a summer break now? But if its been a while since you studied last, you might not realise just how much energy your brain uses while studying or even when starting a new job. Even if you physically might not do as much, it feels like your body is using double the amount of energy because you feel so tired and mentally drained, and maybe even physically drained of energy. Give your body time to adapt, usually during the first few weeks of new school or new work or just new routines in life, dont make exercising the first priority. Instead do other "Life things" such as socializing, exploring, resting or studying/preparing/organizing. If you enjoy exercise you will find time for it again, but usually in the beginning or something new you might need to just take a break from exercise to find yourself, regain energy and then later on find time for exercise in your schedule.
Life is about so much more than food and exercise and weight. Now that you are back in college try to focus on your studies and all the college things and try to live life - as much as possible while still getting school work done, so to say! But what i have found in my own life is that the more i live life, the more i focus on my goals and doing fun things and creating a life i love. The less focus on my body, food or exercise or weight.... those things just arent important if you live a life you enjoy and focus on life.