Human nature is to put things into terms we can understand; we often seperate things into two extremes for example. Good and bad, dark and light, ect..
We simplify, pick things apart, and then put them back together conceptually once we understand what they are in their simplest comprehensible form.
Because of the way we percieve things I believe that we can't comprehend eternity because we don't percieve it 'correctly'...
...yet.
But I have another belief that if it can be concieved of it can be done, no matter how abstract and counterintuitive, it could happen through force of will and time.
The inherent problem with this question ('Is there such a thing as forever?') is that we percieve time as a linear process, a line, it goes in one direction and stops for nothing that we can comprehend. All we can do is learn from the past, react to the present, and guess for the future. For all we know time exists in some incredibly simple and tangible form when percieved by other intelligent beings (don't worry, this is the only part of my theory that's based on aliens

). But just as a dog has a better sense of smell than us these time sensitive aliens can understand time better (percieve it better) because of some need to be able to understand it in such a way (just as said dogs have evolved a 'higher' form of smell because of a need for it). In fact at some point everything percieve has to be put into terms that we can understand or we wouldn't know of it. Therefore I believe that time
is (or can be) tangible
and manipulated. Our existence; our experience even, is all based on what we take in, if our minds existed beyond perception and learning what would they be? Is an existence with nothing to understand existence? Is it at all possible that a human mind could become anything despite a lack of any form of input? So why would time be any different from the other things that a closed off (from perception) but autonomous mind couldn't understand?
I think the answer (to the original question, not my own) is that we must at least ackowlwedge that eternity exists, even if it is only in our imaginations at this point. Once we better understand this universe (who knows what may exist beyond time, space, matter, or energy?) we can put time into terms so that we can percieve it as we would a single object, we would be able to pick it apart and put it together in a way that suits us just as we could a bunch of legos. In fact, that's a good analogy; if time is a fundemental building block of existence that we percieve as a line, maybe the answer is to rebuild the building block, perhaps the answer is to literally pick apart time and put it back together in a way we can understand. Maybe in this sense our perception of time is so flawed that the only way to understand it is to drastically change either what we are or what we're
trying to percieve. What would changing time do to the aliens? Maybe that's why we can only percieve it as we do...
...maybe the aliens got to it first and made it 'theirs'. Maybe time's an incredibly rare element throughout all of everything (even beyond our universe) so it's carefully controlled and maintained, siphoned off into a line and fed to us so we can exist, and the aliens that control it get to decide who gets what, therefore they can do more with it to better understand it.
There are far too many possible explainations but those are the best I could come up with