These sentences are almost identical in meaning. However, in the first, it implies that life is something collective, that our suffering is something shared. I hope that helps.

Our is plural but life is singular so they do not match. Lives is plural so it would work best with our.

As Samantha said, "Our lives are painful" is grammatical correct. (both need to be plural to agree). Does "our" refer to the human race (everyone)? If so, then just say "Life is painful."