When I was an atheist, I used to regard the Christian idea of Heaven as just an eternal version of the hedonism of this world.
In other words, I did not distinguish Heaven from the mainstream idea that the main thing in our lives is to be as happy as possible/ suffer as little as possible; with the mainstream 'utilitarian' notion of morality as wanting the same for as many other people as possible.
But if happiness is made the main thing about Heaven, then this leads to all sorts of difficulties in conceptualizing Heaven - which then make it even harder to believe in the reality of Heaven.
Many Christians already make the error of regarding Heaven as 'perfect in every way' which implies it must be a static state. However, if perfect happiness is also required then this rules-out free will (and indeed individuality) because we must be Made Happy.
And it rules out our participation in creation.
Yet, if we want to summarize Heaven in a single concept; that concept should not be happiness by creation - taking account that creation is inseparable from Love.
The idea of eternal happiness paralyses any possibility of creation - because it removes any motivation for creation. It also paralyses actual interpersonal Love as the characteristic of Heaven - replacing it with an abstract and unchanging love this just-is: going nowhere, because there is no reason to go anywhere (it being already-perfect).
Therefore, we should - I think - take care not to place happiness or any other 'emotion' at the centre of heavenly life; but instead regard happiness, suffering and misery as being subordinated to Heaven's primary reality and goal of loving-creation.
In Heaven we may be happy at any particular time - but we may also suffer. For example, we may aspire - we may desire change which is improvement; and in general be discontented with how things are at present.
For example; there may be - at any given moment and for a particular individual - be unrequited personal love in Heaven. We may wish to become eternally married to someone who does not feel the same way about us. And this is extremely painful: here and now.
But in Heaven we have discarded sin; and therefore unrequited love is a starting point - not an endpoint. In heaven, one who suffers unrequited love will live eternally; but that person will be experiencing, creating and learning - and through this will come healing.
Also, nobody in Heaven is bereft of love; everyone will be in a loving family. And while family love is not the same as love of a spouse; it is certainly far better than the indifference and neglect that so many people experience in mortal life.
Therefore; while Heaven is much happier than mortal life for many people, much of the time; as it ought to be on earth; happiness is best considered a motivator, a guide, a reward - and not the end-point. We will surely feel very happy sometimes, and less happy or miserable at other times.
But we ourselves will be far better able to cope and make the best of these feelings - which will be the basis for our living and working.
Heaven is not about happiness primarily - but about living in love, fully aligned with God, and participating in the divine work of creation.