For instance, what if in a week a asteroid hits earth and kills everyone off? What if aliens come and kill us all or enslave us? What if I wake up tommorow with Cancer? What if I have another panic attack?

Luckily I don't have these thoughts 24/7 but they are still rather common, how do you deal with the unknown, more specifically what the future holds?

Yes I know think positive and everything will be OK, but life isn't a fairytale.