Existence in time, really: the present always passing into the future, the flow of seconds passing through us all, meaning that no rare moment of forgetfulness stays, and our subjective brains are stuck interpreting awful hours of deep struggle as a series of stretched out, stressful, neverending events that will nevertheless also turn into but a fleeting memory that feels like it've only taken a second, since the more we grow up, the more we learn to deploy defensive mechanism of becoming numb to a routine of more or less withering conflict. Passage of time means that we die every day. Passage of time means that things said cannot be unsaid. Passage of time forces us to be careful all the time and makes life what it is: a free for all. Passage of time means that our one shot is never to return. Passage of time turns every beauty into rot. Passage of time means you can always be replaced. Passage of time means that the past seems like fiction, and consequences bury you under them.
Add to that the vileness, ruthlessness and darwinism of life, mix it with human competitiveness on an overcrowded world where everyone just wants the pain to stop, lifes destroyed by things beyond their control, and our constantly failing bodies that can drop at any second, spill the uncertainty of connections and the rarity of true love, then add a pinch of being but one grain in a world of billions with billions already buried. With all of this and more you only have the tip of the shitberg that is life. Basically every single answer people to the painful state of being conscious and feeling life for what it is, is: just deal with it and survive, because you're programmed to survive really, we all know it's shit, we all are stuck in this prison where we can only cope with whatever there is to cope, stop bitching lol, better not try to escape, ok?
Well, "ok."