That's a simple minded misunderstanding of nihilism. It is not a license to just do whatever with no consequences or consideration.
Nothing matters, because there is no such thing as divine law or morality, so it doesn't matter what you choose to matter to you. That's entirely your opinion to have and dictate your life around. We are free to shape our own reality and morals.
You're right. We aren't special in the grand scheme. Humans are just another animal in the ecosystem. But I am me and I choose to care about myself, so I choose to care about whether I live or die.