I cannot stay at the beach without my rash suit

I'm so shy and I don't like staying at the beach, but I have to go there in summer since most of my friends spend there their holiday. So every year is the same: I'm thin as a rake and I don't like showing my body to others, especially my skin which is so dull.
I work at my office all year around and I do not make sport at all. In summer my friends take me to these aquagym, pilates and fitness classes on the beach. Everybody is in bikini or tank tops, their skin tanned and their muscles in good shape, so I'm used to wear a rashsuit over my bikini and panties. It's long sleeves and legs protect me from sunburns, they keep sand and water outside and make me feel safe under other people's sights.

But yesterday during a postural exercise class at the beach, the instructor, a young lady around 30 with strong muscles, walking between us to check and help our trainings, asked me to stay in my bikini like all others because my rash suit prevented to see if my shoulders and my back were being trained in the correct positions.
Other girls heared that and laughed saying I was a bit strange with my rashy on, since it was not cold at all under the sun. The point is that I felt really confortable and protected in that wetsuit.
Anyway I tried to meet the instructor's request and unzipped a short part of the front zipper in order to make my shoulders visibles.
She said I should really sunbathe without that clothing since my skin is so dull. After a few exercices she came back to me, she opened my zipper to the bottom and stripped my arms out of the sleeves. I felt so exposed without the upper part of my rash guard.

During the day some girls and my friends threw sand and water against me saying that finally I was no longer protected by my rashguard, I felt those cold drops of water over my dull skin and my zipper was completely open and swinging around my waist, completely unable to protect me like before.
Direct sunlight and contact with the sand were so strange and unconfortable to me and later I had to take my rash suit out of my legs too and left staying in my bikini. They laughed, said "urraahh!" and gave me some suncreme, but it's completely different from my wetsuit.

A girl looked inside my beautiful rash suit when I took it off and read aloud a label inside that which said "rash guard designed to protect yourself from sunburns, scrathes, wind and cold with a strong barrier around you" or something like that. Then they laughed and said "say goodbye to your shield for today", they closed the zipper and started filling my suit with sand and shells, making it totally useless.

The instructor told me that now I have no excuses and need to sunbathe and do sport since my body definitely needs it now that is no longer under a rashie.
Now I don't know whether I should go back to the beach again with my friends oe staying at home on myself.