I'm sorry, but for me an overweight person in a color guard can be a glaring distraction. Color guards are become much more like professional dance units - looks are very important. It's the way it is. If it's your dream - lose some weight and go for it again next year. I was 240 pounds in 2000. I decided to go to the gym and do something about it. Within about 9-10 months I was down to 180. It can be done. STOP MAKING EXCUSES!!!!! It may sound harsh, but I have done it myself - for years struggling with weight. Believe me, when you drop that first 10 pounds, you will be thrilled. When you drop 20 pounds, you will be elated and nothing will stop you. It is much easier than you believe. Exercise 5 days per week and watch what you eat!
The problem is people in the U.S. are brought up not to say things like, "Hey, you're fat!" It took my German friend who said, "Ok, I have to say, you have gained a lot of weight over the past two years and are looking fat" to get me to do something. I was shocked that one of my best friends would say that....but I'm so grateful that he did, because for the first time I thought, "Yeah I am fat!"
Join a gym, limit your caloric intake and exercise. No carbs, no fat, no sugar, South Beach, Atkins - they are all fads. It's simple math - burn more calories per day than you intake and you will lose weight. Of course, taking 200 calories from fruit rather than ice cream is a better choice because ice cream is so fattening - but just figure out how many calories someone your size burns per day and make sure that at the end of the day your net calories are less than that number.