Hi.
Summer can be hard if you're body-conscious--which most people seem to be. The standard advice is to just relax, but sometimes that's not enough.
One idea is to try and shift your focus from these feelings about your body to the waterpark itself and having fun with your friends. I suspect that seeing all those people in bathing suits around you will make you less self-conscious anyway, especially once you all get wet! It's pretty unlikely that anyone will have anything to say about your appearance--they'll be too concerned with their own suits...
But my main advice is to just give yourself a break. Learning to feel comfortable with your body can take some time. If you decide to wear a suit to the waterpark, you might want to wear a t-shirt or something over your suit at first. And then just see how you feel.
The decision about whether to go to the waterpark and what you wear is strictly yours, of course. (You do say in your letter that you "really want to go.")
While you're weighing your options, however, you might want to check out some of these resources about body image.
