I have this mole on my arm that has the ABCD's of melanoma, but doctor said it was fine?

Definitely get a second opinion. My doctor insisted I was fine for two years when I had cancer. By the time we caught it, I was stage 3 and had three tumors that had metastasized. I wish I had listened to my gut and got a second opinion. Melanoma is especially tricky. If caught early you have over a 90 percent success rate, if not then it drops to 15 percent. I wouldn't risk it. My doctor is proof enough that they don't all know what they are talking about.