There is simply, nothing.
Trust me, I'm a former Christian, and I'd love nothing more to know there's an afterlife. Not just for me, but for those I lost. For my father, for my wonderful 26-year old cousin, who was the kindest soul I ever knew, who died of cervical cancer that developed when she got pregnant and had a child, and left so much in shambles, for my 19-year old friend who fought in Afghanistan, and sacrificed so much just being there, and being outright hated by well more than half his country (gay), who bled to death slowly and painfully after being shot rescuing one of his comrades during an ambush, for my 16-year old friend who was abandoned by her father because she was transgendered, whose mother began emotionally abusing her because the mother was driven to alcoholism over the loss, that girl who died without ever feeling complete, having spent every moment of her life broken, and then froze to death in some street alley.
I'd love nothing more for there to be an afterlife, not even for me, but for them.
And that only reinforces why I don't think it exists.
We're not only afraid of our own deaths, but of those of our friends, our family, those we hold close, those whom we esteem, and those we admire. Its emotionally torture to not only lose them, but to realize they've lost everything we still have. It makes us feel weak, fragile, and only more sad for their departure. To know that such a life will never exist again is soul-crushing. And thats reinforces why I think heaven is a delusion. We rationalize that pain away by saying "heaven/a-life-after-this exists", it helps us cope and keep from our soul being crushed. It was painful to write that about my lost loved ones, but death isn't beautiful. Death is ugly. Death devalues and trivializes our life, our existence. It reminds us that we are flawed, that we are merely mortals. Belief in the afterlife is an attempt to dodge that otherwise emotionally-crippling pain.