No purpose if we cease to exist after we die? Is not leaving a better world for our descendants not purpose enough? Is not making life better for our fellow humans not purpose enough? What is it with Christians and their "if humans do not matter for eternity they do not matter at all" sickness?
You are a member of a very unique species: a species able to define a purpose for itself. Nature spent 13,500,000,000 years creating a brain capable of this unique task. Honor the effort and use it. Or, wallow in your nihilist mental squalor. It's up to you.