I can understand your fear of cancer considering the fact your mother had it, but I also think you're worrying a bit overmuch at the moment. You saw a dermatologist who did a thorough exam on the mole and he obviously didn't think it was anything of major concern for the moment, and he is a professional. Moles tend to show very clear signs if they are becoming cancerous; there are almost always very obvious differences in the shape, size, color, etc. Bottom line: your derm has seen a lot of moles in his line of work, and he thinks your mole is normal, so it probably is. It isn't likely that your mole would become cancerous without you noticing anything strikingly different about it. Try to stop picking at it, and after the pain subsides, maybe your worries will a little as well.