Why does my melanoma keep returning?

I have been diagnosed with skin cancer. None have been the worst kind. But once I have one removed, another pops up. At the present time I have at least twenty on my legs. The dermotologist has removed many and sprayed many. She says they came from sun damage. I can't understand how they keep poping up and some come back. I am discouraged and dont know where else to turn. Can you give me an opinion? Faith