To shy to wear shorts advice?

That approach by your mother doesn't sound right to me. Especially at your age, it sounds like she wants you to get attention from the boys. I was leg shy at your age, but I love to wear short shorts, now. Just wear them for short periods of time to get used to them. It probably would be OK for you to wear regular length shorts, not short shorts, start with those, then wear shorter ones later, if you feel comfortable.