What Should Have Happened ...

 The first thing to do is admit that it went wrong.

The next thing is to figure out what should have happened instead.

With the two outcomes, there will be a bunch of points in time where things deviated from the desired path. Find all of them, list all of them out.

In some cases, an earlier deviation invalidates some of the later ones. That is, if it had not initially deviated, then some later deviation would never have occurred and is rendered moot.

That leaves you with a list of ‘critical’ deviations. They are independent, or at least mostly independent enough that they all matter. 

Keep in mind that a bunch of concurrent small problems can interact together to create much larger ones. If one or more of the deviations was a ‘perfect storm’, then you need to list out all of the other contributing littler deviations.

With a final list of significant deviations, you can roughly assign a weight and fixing costs to them. 

An earlier deviation might actually have very little weight, it might just be more of an annoyance. But it still should be documented as ‘contributing’. 

There might be some deviations whose real cost is far too high to realistically fix them. The only solution for those is to put in place controls or mitigations to reduce their effect next time. But they should be noted as expected. 

So now you have:

• What went wrong.

• What should have happened instead.

• A full list of everything significant, including breakdowns of lesser contributions.

• A list of alternatives and a list of mitigations. Including the approx weights and costs of each.

The only thing left to do is march through the list and fix things.