I totally understand what you mean I feel exactly the same, I can't come to terms with the fact that I will cease to exist, not for me but for my children.
I know that the time they will need me the most in their lives I will have died. I hate that thought BUT I have started to prepare a memory box, I write them letters once or twice a year and seal them and put them in the box, photos, trinkets from days out. I know that they will have memories and new communication with me after I have died and that helps me sleep a little easier.
We're all in the same point and cannot do a damn thing about it so just prepare for it and experience as much of life as you possibly can. No regrets :)