After a breakup or divorce

The canon has no chapter on divorce, but it is plain about separation from what is dear: it names the loss of everyone and everything we love as something no one escapes, and it tells a grieving grandmother that a hundred loved ones mean a hundred griefs. Nothing in that asks you to feel less. The suttas also know the other half of an ending — the wanting that keeps checking, the story rerun at night — and one short set of verses advises against running after a past that has already gone. What they cannot do is say whether the relationship should have ended, or how to divide a life. If there are children, custody, or safety questions, those belong with people who can act.

What the suttas say

“Those who have a hundred loved ones, Visākhā, have a hundred sufferings. Those who have ninety loved ones, or eighty, seventy, sixty, fifty, forty, thirty, twenty, ten, nine, eight, seven, six, five, four, three, two, or one loved one have one suffering. Those who have no loved ones have no suffering. They are free of sorrow, stains, and anguish I say.”
Visākhā's grandchild — the Buddha's plain acknowledgement that love and grief come together.
Where this stops: Not a counsel to love less; a description of the terms on which we love.
Ud 8.8:5.1–5.20 · With Visākhā
Don’t run back to the past, don’t anticipate the future. What’s past is left behind, the future has not arrived; and any present phenomenon you clearly discern in every case. The unfaltering, the unshakable: having known that, foster it.
The Bhaddekaratta verses — letting go of the chase after what is gone and what has not yet come, in favour of what is here.
Where this stops: The verses go on to urge effort today, since no one knows whether death comes tomorrow; they are about where attention goes, not a dismissal of planning or of a past that still needs repair.
MN 131:3.1–3.8 · One Fine Night
Ānanda, did I not prepare for this when I explained that we must be parted and separated from all we hold dear and beloved? How could it possibly be so that what is born, created, conditioned, and liable to wear out should not wear out? That is not possible.
Ānanda's grief at Sāriputta's death, and the Buddha's answer.
Where this stops: Never offered to hurry grief, and never use its point about impermanence to tell someone their loss should hurt less.
SN 47.13:6.1–6.4 · With Cunda
When a man lives heedlessly, craving grows in them like a camel’s foot creeper. They jump from one thing to the next, like a langur greedy for fruit in a forest grove. Whoever is beaten by this wretched craving, this attachment to the world, their sorrow grows, like grass in the rain. But whoever prevails over this wretched craving, so hard to get over in the world, their sorrows fall from them, like a drop from a lotus-leaf.
Wanting that will not settle, in its most general form: a person who leaps about like a monkey after fruit, and the plain statement that sorrow falls away as craving is overcome.
Where this stops: About craving as a habit of mind, not about any particular wish; it is not a verdict on someone who wants a better job, a fair share or a child's welfare. Where a habit has become a dependency, it is for a doctor or a program first.
Dhp 334:1–336:4 · 24. Craving
Mendicants, there are five things that cannot be had by any ascetic or brahmin or god or Māra or divinity or by anyone in the world. What five? That someone liable to old age should not grow old. That someone liable to sickness should not get sick. … That someone liable to death should not die. … That someone liable to ending should not end. … That someone liable to perishing should not perish. …
That what ages might escape aging — the impossibility named without softening.
Where this stops: Never offered to hurry grief or to tell someone they should stop grieving.
AN 5.48:1.1–1.4 · Things That Cannot Be Had

Bring your own situation.