I dont know how to accept it

My dad died unexpectedly of a heart attack 6 years ago while I was living overseas. It devastated my mum and sister but we just kept going because we didnt know what else to do. I'm sitting here 6 years later thinking that I'd give anything to say just goodbye to him. I dont believe in an afterlife so I'm finding it really hard to just accept that hes gone. Or rather I know hes gone but I dont know how to move past it. I don't know what I'm looking for in posting this.