Can someone help with my anxiety?

I have anxiety. But I'm not sure what causes it. I've tried finding different things that could trigger the nervousness, but I haven't found anything! I get really hot, I get really nauceous, my stomach feels 'gross', my breathing becomes harder,I feel dizzy and I clench my teeth and I kinda shake. It sucks. It happens when I'm outside of my house, or even if I'm in bed! I have no clue whats going on. Its getting harder and harder to get through school. I have trouble falling asleep, even though I take Melatonin to help me. I've been to the doctor and they think I have ADD, which may make me focus more on my anxiety than anything else that's happening. Can someone please help me on how to relax and actually enjoy myself?