I'm painfully self conscious about my body?

Hi... well I'm 18 and I'm dreading summer again. I'm ridiculously self conscious about my body - I don't hate it, but just the idea of showing it or other people looking at it fills me with absolute dread and I feel like I want to be sick. The thing is, I'm very very slim, like the biggest my waist ever gets is 24 inches, its usually around 23/23.5, narrow hips, and I do quite a lot of exercise, my legs are nice, my friends and family always say I've got a lovely figure but I just have this debilitating fear around it. Most of it is because I don't have a flat stomach - I mean its not huge and it looks pretty flat under clothing but it isn't. I've always had this, well the best word to describe it is phobia of it. I feel like I can never wear a bikini or anything like that because it's not perfect. Even in normal clothes I keep my jacket on because I don't want anyone catching a glimpse of it, and sitting down I arrange my clothing in such a way so that you can't. It's not that I hate it, I don't!! But everyone says I'm so slender so if I let them see my body they'd realise I'm not perfect - I know it is shallow but I've always been 'the skinny one' so I'm scared of letting anyone realise that not every body part is perfect-looking. I don't even let my mum see, even though she's always trying to get me to show myself off more, because I feel like even she would judge me. Everyone always says how gorgeous she was when she was younger. I don't like eating junk food because I don't want my stomach to get bigger and put on weight. The stupid thing is I know I'll regret it when I'm older for not showing myself off when I was young enough, but it just paralyses me with fear at thinking of it. What do I do??? =[