Surely, our past and future aren’t always better than the present. Yet we continue to think that this is the case.Well, it's always good in a discussion to first get your terms in order. Eternal means timeless. So happiness all the time does not mean the same thing as eternal happiness. So most of what is discussed here is beside the point.
These are the bricks that wall off harsh reality from the part of our mind that thinks about past and future happiness. Entire religions have been constructed from them. Whether we’re talking about our ancestral Garden of Eden (when things were great!) or the promise of unfathomable future happiness in Heaven, Valhalla, Jannah or Vaikuntha, eternal happiness is always the carrot dangling from the end of the divine stick.