The following are things I learned, which at the time were well sourced. I did not make up any of this! I do not remember the sources, but perhaps some of the more learned people here can identify those sources:
Another approach is that Gehinom is an experience of intolerable deep shame and regret.
In this world, we are subject to time, as well as other parts of creation that mask the perpetual reality of Hashem, His Light, and our only purpose for being here – to do things that will make us closer to Him (read: s’char – reward). We have opportunities here. Once our time is up, we separate from the limitations of our bodies, creation in this world, and all other “things” that only exist in the world as we know it when we are alive. Those things may have served as a mask to the full Glory of Hashem, and His Omnipotence, causing us to at times choose things that did not bring us closer to Him. All at once, (there is no time outside the creation that we live in), we are faced with where we ARE, compared to where we COULD have been. It is crystal clear to us. And we can no longer do anything about it. The shame and regret is beyond what a living human being can fathom.
note: those of us in this world, who were in ANY way affected by the Neshama that has departed, CAN do things HERE that can affect that Neshama. The mitzvos, Torah learning, Kiddush Shem Shamayim, etc. that we do has a direct impact on “compensating” for the lost opportunities. That is why we say Kaddish for the first 11 months, and why children, students, or acquaintances deliberately do things for the Neshama to have an “Aliya”. After the first year, the yahrtzeit serves as a time where the persons’ life is again under “review” of some sort, and is a date on which we can, and should do things that give the Neshama an Aliya.