Since our consciousness, which is what lets us feel "alive," is a pattern of atoms that constantly changes, I think once we die it just breaks and the essence of "you" disappears. It lasts forever, but since you are no longer there to perceive it, it may as well be instantaneous. If I'm feeling a little more fanciful I might consider that there is perhaps a soul, or something similar to it, that gives us our consciousness and awareness, and when we die the connection between our soul and body breaks. At that point I can't begin to comprehend what there could possibly be, but perhaps without a tether we just melt into the plane of existence that it lives on. Ultimately, we can't know! It's just beyond our comprehension. Fun to think about though!