Looking back on life, everyone has regrets, b. But when we know that we have things to regret, the things have already gone.

The sentence is very long, I separated it into two sentences for clarity.

It seems like we can'’t know that we have things tosee our regrets while the things are taking placeit is happening, which is the funny part of life.

This sentence sounds a little awkward, so it is hard to understand what you are saying. I don’t know what “things” is in this sentence.

“we can’t see” ==> sounds more natural than “know”
“our regrets” ==> I changed “things to regret” to “regrets” to make it easier to understand. Regret can be a noun or a verb, so I used it as a verb in this sentence. The “things” that people can regret is not as important or clear as the “regret” in your sentence, so that is why I removed it.
“while the things are taking place” ==> It is not clear what are the “things” and what is “taking place”. (I know you mean “things to regret” but it is not clear what is being regretted.) I changed it to “while it is happening” to show that we do not see what we will regret in that period of time. You could also change “while it is happening” to be “in the moment” instead, but I don’t want to change your sentence too much.

Looking back on life, everyone has regrets, but whenby time we know that we will have things towe regret, those things have already gohappened.

It seems like we can't know that we have things towill have regrets while the thingsy are taking place, which is thea funny part of life.

a instead of the. In this context, the makes it sound like it is the only funny part of life.

It may also be thean amazing part of life at the same time.

An instead of the. Same reason as the edit above.

NeverthelessEither way, it's not a big deal.