Smoking doesn't mean you've mostly killed yourself. You can quit and regain your health.
If you like to read, I recommend The Five Invitations by Frank Ostaseski. It's a look at facing your own death from a Buddhist perspective. Also When Breath Becomes Air by Paul Kalanathi, a man who knew he was dying when he wrote the book. I have found that learning how to face your mortality instead of ignoring it is much more comforting.
As for an afterlife, there is zero reason to worry about that because we have zero information about what it would be, if there even is one. I live my life the best I can and trust that if there is something after, I can react to it then.
This is the kind of ridiculous shit I come to the internet for.