Even though our culture will try very hard to convince you otherwise, your worth as a person is NOT determined by how you look. As you age, your body will change. Thats just a fact of life. Your body is just a shell that you will one day shed and will end up in a grave somewhere. True beauty and worth comes from who you are on the inside. Some of the ugliest people are beautiful on the outside.
i used to obsess so hard about how i look, to the point that i would cry regularly about it. as i got older, i started to realize that how people look becomes the last thing anyone thinks about... and anyone who is obsessed with looks to the point that they feel a need to judge you is simply immature. you deserve the right to simply exist as a human being. the concept that you always have to be attractive "or else" is a societal concept that was constructed out of capitalism, classism, and sexism. you're allowed to look however you look. next time you go out somewhere, don't stare at the celebrities on magazine covers who spent hours if not days being made to look like that for a single photo. look at the real people around you, who are just living out their daily lives. if you decide to exercise and eat healthy, you should do those things because it is good for your body and your brain! exercise and healthy eating will help you feel better mentally and physically. but don't do it for the lone purpose of making yourself look better, or it will drive you crazy searching for immediate results.
The way we consider beauty and the value we place on it has been heavily manipulated by our culture. In some ways, I fit with my region's standards of beauty, and in other ways, I don't. Which is fine, because unlike a lot of models on magazines and in advertisements whose appearance has been heavily modified and touched up using modern technology, I'm not the equivalent of a computer-generated image. I'm a human - my face and body tell a story, and they are unique to me. I don't have to look beautiful, because I am unique and I am myself, and that is even better. I look beautiful in my own way, and other people look beautiful in their own way - beauty is subjective, and in the eye of the beholder. We may share some things in common, but in the end, all of our faces and bodies are beautifully unique. I would definitely recommend exercising and eating well, but as pamela97 pointed out, do these things to feel better. Don't attach them to your self-worth.