I think the enamel has came off my teeth!

I used to have a beautiful smile. Really white teeth, and perfectly straight. I am only 21, and I think the enamel has came off my teeth (or it's in the process of coming off). I'm not sure, but this could have been caused by brushing too hard. I never knew this could happen, until it was too late. It all happened pretty fast. I have started brushing really lightly, but it doesn't seem to be getting any better. I am so upset about this, it has made me half sick. I have noticed that my teeth aren't as thick as they used to be. They are also discolored (more yellow on the center of most of my teeth), some moreso than others. Also, on my two front teeth there are really white "scrape" marks in some spots. My teeth are starting to stick to my mouth as well. Is there something that can be done, to make the appearance of my teeth look like they used to? I have a regular cleaning appointment at the dentist office in about a week, and I am going to ask about all of this. But, I'm not sure I want my teeth cleaned, if it is going to damage my teeth even more! Because if I took my finger nail and touched my tooth with it, I could take a chunk out of about 4 of my teeth. Please share any information you can. I am becoming very self-concious because of this! I don't want to smile anymore!