You could make a shopping trip to buy a formal outfit that is also comfortable. Feel all the fabrics before you try them on, check for itchy tags, make sure it fits and nothing is too tight. If, for instance, you find the waistband of slacks uncomfortable, look for pants with elastic or a draw string. If you're a guy and you feel like you're choking when you wear a tie, buy a banded collar shirt that doesn't require one. If you're a girl and pantyhose and heels bother you, get some cotton blend footless tights and some ballet flats.
If you have a diagnosis and your parents know about your sensory issues, they should be willing to go shopping with you to solve the problem. If you haven't communicated to them what it is specifically about the clothes that makes you uncomfortable, they probably think you are just not wanting to go out to events or you are being afraid of change when you have to wear something besides your everyday clothing.