First of all, atheism does not mean a lack of belief in an afterlife, it means a lack of belief in a god, nor does atheism imply that life is worthless or meaningless. What AlbanianWhelk is describing is 'nihilism'.
Secondly, I'm an atheist, and thought I believe in neither a god nor an afterlife, I feel exactly the opposite.
Neither my, nor anyone else's, life is intrinsicly worthless because that one life is all we have. No second chances, no do-overs, no 1-ups, no respawns and no overpowered fairy king keeping score and grading us afterwards.
Neither my, nor anyone else's, life is intrinsicly worthless because, for good or bad, your life is what you make of it. Though there is no god keeping score, at the end of my life it is theoretically possible to tot up all the positive and negative influences I had on others and on the World. It won't matter to me or course, I'll be ao much unthinking, unfeeling meat, but I'm descended from the apes, a highly social order with a propensity for building and maintaining strong group bonds and for behaving altruistically and empathically.
I want the lives of those I leave behind to have been better for knowing me, which is neither worthless nor meaningless. I also do this knowing there is no eternal reward waiting for me, because I want to, not because a book tells me I must.
