Haven't worn a bathing suit in over 6 years.. help?

I'm 16, and I'm soo insecure of my body. I guess I have a pare shape? My thighs, butt, and hips are really big. My measurements are 30-26-41. I'm 5'5", and I weigh 134. I have stretch marks (most of them faded, but still noticeable), and I have cellulite in areas of my butt and legs. I'm always wearing long sleeve shirts, and long pants. I don't think I've ever worn shorts before. My mom says my butt is like a bowl of jelly. I've tried exercising, but the only thing that gets smaller are my waist, and breasts. My friend invited me to her birthday at the beach again, and this is the third year I'll have to turn her down because I'm too scared to show my body. I'm scared people will make fun of me, or look at me with disgust. What should I do? I live in California where there are tons of beaches and sunshine, but I stay inside and hide from it all. Any advice? Thank you!