Why is my confidence wavering so much?

Ok so I used to love singing and I still like singing but feel that I am not good enough anymore. Singing is my only hope but I am finding it difficult to focus on it much because my mind is constantly foggy. However when I do sing I start shaking like a jelly, even if I am by myself and I don't know why. I just want it to be perfect. If I do sing, I practise for maybe 5 hours or more on the same song until it is absolutely perfect. I really fancy going to karaoke but I am too scared to go as I have no friends and am really shy around other people... what do you suggest?