I hear so many people talk about after death basking in eternal bliss in the presence of their deity. It seems to me that they just really haven't thought about it. Or, maybe I over-think it. Eternity means that time is no longer relevant, which necessarily means that space is no longer relevant since the two dimensions are tied inextricably together. If time and space don't exist in eternity, then there are no physical differences that identify an individual as a unique soul. Even thought itself requires time, because thought happens sequentially. So, all that's left is an idea of the individual,... but whose idea is it? Here's where it gets complicated.
Just a few years ago, I realized that I had stopped thinking of God as an independent personality, separate from me, and to whom I could turn for guidance or an occasional favor. I found that my perspective had changed almost imperceptibly, so that I no longer felt like a created being, but an emerged being. I now think of God in terms of All That Is, and that I am the sensory organ that gives God its viewpoint on itself from my particular perspective. God experiences itself through each of us, and is so pervasively infused with all of us that saying a prayer is not much different than talking to yourself in a focused frame of mind.
Given this, how is it possible to seriously consider "spending all of eternity in the presence of God"? Being in the presence of anything implies separation. So, Eternity is not a concept with any merit for someone who wants to hold onto his or her own separate, unique, individual soul. Therefore, an eternal Heaven (in the sense that most people think of it) is not possible. At best, an angelic Heaven could last eons, but Time and Space must be a part of the picture.
Even so, it's still not reasonable to believe that anyone could remain in static ecstasy for longer than a few moments. Many strong believers I know describe Heaven to be something like gazing in eternal bliss at the face of God (or Jesus or some representation of Deity). This simply cannot happen for all eternity. There MUST be fluctuations or differences in experience, if an experience is to have any definition or reality at all. In order to maintain a sense of identity, the identity must be expressed in contrast to its spiritual context or environment.
We each see ourselves as the leading edge of our own individual journeys. We create our unique identities as we go along, with each new experience and with the effects of each new choice. When my body dies though, and I stop creating my identity, it won't die right away. The relationships that defined me in life and that define me in memory will maintain my sense of self for as long as it matters. And then, it just won't matter any more.
So in a nutshell, there is no eternal existence, but instead, we continue to exist for as long as knowing that we exist really matters. Our reality is both Heaven and Hell, and the fun lies in expressing our Selves to make a lasting impression that we can be proud of. It will last as long as it is expressed in some way.