It will be fine! You are so young! You have the rest of your adult life to enjoy fit, slim and healthy! From the perspective of a 47 year old women who in her head still thinks she mid to late 20's (though hasten to add I am not mutton dressed as lamb!) you have been an adult only for a handful of years!
Seriously the changes you are making now and the exercise you are incorporating will make a huge difference in the years to come because you are making a lifestyle change whilst still enjoying calorie laden food now and then. I honestly think in the future you won't ever have to be on "diet" as your "diet" (as in what you eat) will be informed choices you make naturally - like naturally slim people do (well that's what my slim friends say and I guess I used to before I started overeating in my late 30's.
You head will soon catch to the changes in you body when you catch a glimpse of your reflection in shop window and think who's that slim young man! I'm very excited to see read where you are and how you will feel in December/January.