Clinging to the past might manifest as a resistance to change, a rejection of impermanence (cf. the heart of sadness (2/3) — separation from the loved). Accepting that some change has occured, that the past is ‘lost’ and cannot be reverted to (if only because ourselves have been changed by the experience), is an important step in reaching the heart of sadness, in realising the unsatisfactoriness of life, in fully comprehending dukkha.

But can we go further? To attain the very ‘heart’ of sadness, one needs to dig deeper.
I'd argue that a relatively classic unfolding of the “separation from the loved” leads to the perpetuation of dukkha even after the grieving is said to be completed, even after one has apparently accepted the separation, the lack of reality of any “return scenario”… I'd argue that the ordinary unfolding of the “separation from the loved” leads to indirect pains too, secondary and tertiary consequences… And as we might, through life (or lives), accumulate many such indirect tendencies if we perpetuate them unconsciously, it is worth enquiring further, it is worth going beyond the ‘obvious’ primary aspects.

Lingering taste

After one grieved a loss, the acceptance of the new situation might simply be related to understanding that we never step in the same river twice, that even if one could be reunited with the loved, in fact, one would have been changed by the experience of separation and this would not be exactly the same. A couple can be reunited but the blind trust might be gone: maybe a new form of trust is created, it might even be better than before, less naïve, but it isn't “the same” anyway.
Or the acceptance might be related to a loss of hope, the relinquishing of scheming and planning (for a reunion) and the appropriation of a new view (that the reunion is now impossible, for some reason or another).

In either cases, one might still cling and suffer though.

“Self-discovery”, from the “Adam&Eve” series, © Pavel Kiselev

The ordinary occurrence is when one tries to re-create the ‘essence’ of the loved even after accepting that the previous ‘form’ is lost!
Classically, this may take the form of having accepted that a person is gone but trying to recreate the intimacy that was once enjoyed with another person. Or this might take the form of accepting a country is invaded and destroyed but trying to recreate somewhere else a sense of ‘roots’ that was once enjoyed.
In short, the suffering is perpetuated by shifting from a particular object to its supposed essence, its supposed ‘core'… The clinging is refocused on a particular detail that supposedly made the previous relationship worth it, and this detail is then what one seeks to find again, within another relationship, another object: different person but same intimacy, different location but same culture…

It is well understood that “rebound relationships” are often doomed to fail because one still clings to, or regrets, some element of the past and cannot value or appreciate what's actually present. This relates to the above.
But how many people who understand this vis-à-vis rebounds would, however, engage in their present relationship while clinging to views from the honey-moon period?

Day-1 is lost: you have changed, your partner has changed. Now what? Are you with the ‘same’ person but in a “rebound relationship,” constantly trying to go back to the (day-1) past, to the enthusiastic first kiss, denying the obvious change in circumstances? Are you labelling your partner ‘husband’ or ‘wife’, at the risk not only of taking them for granted but also of limiting them? Or are you with the same person creating a new love every day, a new day-1, unbiased by views from previous days? As I previously explained, Buddhism doesn't ask you to stop loving your family but this precisely requires to love who they are now, neither some memory nor some projection! Projections are to be ceased; loving-kindness, compassion and sympathetic joy aren't.

Separation from the loved is stress.
You cannot get the loved back, and you cannot re-create it either… You might create something else new, but comparing it with the previous loved will bias judgements and most likely be enough to cause unsatisfactoriness (due to the unease of having to drop old certainties and convictions, to adjust one's views and expectations). You have to swallow the primary change, but you also have to let go of the lingering taste, the lingering preference…
It may sound cold but it isn't. Life simply isn't about accumulating. Love isn't about how many days you spent together: are you loving, kind, compassionate here and now? This is the relevant question. Drop the delusion that “more is better,” remember that you cannot carry the accumulation anywhere. Old love change? Let it evolve, focus on what you contribute to what arises next!

As I once explained, Buddhism does not assert that “life is suffering”, in spite of all the classic translations which suggest so. This wouldn't make much sense, and would be wholly disconnected from the second noble truth. “Life is pleasurable and ordinary minds can't get enough of it” is more like it: ordinary minds crave! However, if we phrase the message in terms of ‘truths’, it seems safe to state that “life does not move according to one's wishes.” To realise the heart of sadness requires to realise that our wishes are past-based, created in relation to experiences, and that changing how we relate to this past will also change our experience here and now. Not-clinging to the past (which isn't the same as developing aversion for the past, or as denying it) is key to engage freely and constructively with the situation at hand.

“Fully realise dukkha, fully realise impermanence, fully realise emptiness of essence, embody a wise way of life”, these are four tasks the Blessed One completed. Nirvana is unchanging.