I agree with Rick. Your dermatologist's assistant? Is she even a doctor? It sounds irresponsible to me, that she would offer any kind of diagnosis. And again, every melanome I've ever seen is usually black like charcoal -it even has that texture. It might be basal cell, in which case, it can be removed promptly. This spot on your chest is brand new, and it's not like it's been growing there for 2 years.
The good thing is, even if it is a basal cell, it is on your chest, and not on your face. I have a spot that my dermatologist is watching closely, that might be a basal cell. It's right on my cheek, so I have to look at the damn thing every day! But I caught mine early as well, by accident. I went in for something else, and my dermatologist noticed it.
It sounds like you may have some health anxiety as well. I know, because I often respond like you do -jump to the worst possible scenario and start freaking out. This type of thinking can make you feel worse! Check out healthanxiety.com and see if you can relate. Some people that have health anxiety (like myself) find that cognative behavioral therapy is good. There are also support groups like recovery inc. that have group meetings (I haven't been to one, but I hear that they are good).
You don't want to have a nervous breakdown every time you get a weird spot on your skin. That's no way to live.