I was dxd when I was 6, 18 years ago. I don't know if I gained a lot of weight, since not only was I growing due to my age, but I lost a ton of weight before I was diagnosed. I was never what I would consider skinny in high school, but my weight was pretty stable. After my first year of college, I got more serious about exercising and eating better. I got a pump in 7th grade, which made it easier to eat things I shouldn't, cuz I could just crank up the insulin. I have actually lost about 12 lbs over the last year and a half. Most of this is probably due to increased exercise. I was moderately active in high school, but in college I had a nice gym 2 minutes away, so I really started trying harder and I have worked hard to maintain it.
Some people will gain weight, but certainly not everyone, especially if you try to stay on top of it. You are in a super vulnerable spot right now, this process is so new and the huge amount of information you are processing is overwhelming and scary - all the health problems and "recommended" diets that seem impossible to actually follow... And to complicate stuff further, the rest of your body works just fine, which means all the stress you are under could trigger weight gain as well. For me, high amounts of stress seriously messes with my blood sugar.
As for not taking insulin, as others mentioned, you feel terrible! When I am high, I can't think well, my mind is foggy, I am exhausted and kinda nauseous, my ability to focus is greatly restricted. I can't afford to walk around in haze like that, my job is too important and I would make tons of dumb mistakes or miss deadlines because I couldn't get my brain to work.
Try to focus on one day and one thing at a time. Everybody has bad days, but it will get easier! Everybody responds to things differently, so one person's experience may be very different from yours. You can totally do this!!