Learn English – Corresponding English saying for a Spanish one about misfortunes and circumstances

The closest I can think of is

a gift horse

which references the saying

Don’t look a gift horse in the mouth.

Though it is not a precise match.

A horse’s teeth are often a good indication of the horse’s overall health, so when purchasing a horse it was important to look in its mouth to ensure the teeth looked healthy. But the proverb tells you not to do this with a gift horse: since it was freely given, it does not matter what its condition is. Even an unhealthy horse is fine when it’s free. (The proverb is also given as a rule of etiquette, since it would be rude to question the value of a gift freely given.)

But if this is similar enough, then you could use a gift horse to refer to something you got for free, and therefore did not care how useful or valuable it was since it cost you nothing. It should be noted that while don’t look a gift horse in the mouth is a common, well-known saying, the phrase a gift horse referencing it is not common. I think most readers familiar with the saying will get it, but some might not get it immediately and react with “what horse?” You could give them a nudge in the right direction by saying something like a proverbial gift horse to indicate that you’re talking about the proverb, that this is a horse you shouldn’t look in the mouth.

As an aside, a term very different in meaning is a Trojan horse, which was a gifted horse that very much should have been inspected. It was, of course, a large wooden horse rather than a real horse, but it was famously left outside Troy by the Greek armies that had beseiged it. It appeared to have been left as a sign of respect for their bravery in defending the city, and it was taken into Troy. Of course, it was actually a trick: Greek soldiers had hid inside it, and that night they snuck out of it, now inside the walls, and the sneak attack took Troy. Hence the phrase beware of Greeks bearing gifts.

The fact that this trick took the form of a gifted horse while the proverb about not inspecting gifts also refers to a gifted horse is a historical coincidence, but one that is sometimes used to make a point (for example, about the limits of etiquette, or about the hidden costs of what might appear to be free).

The closest that I can think of is not a saying but an idiomatic expression I know what it's like...

I know what it's like to be out of work... I was on the dole for three years

I know what it's like when you break up... John and I split up three months ago and I'm still not over it.