You lost your temper over something small, and then spent the evening apologising for it. The size of the reaction did not match the size of the moment, and that mismatch is the useful part. Anger that is out of proportion is almost never about the thing in front of you.
What anger is actually doing
Anger is a fast emotion sitting on top of a slower one. Underneath it there is almost always something that arrived first and is harder to say out loud: hurt, fear, shame, or the plain feeling of not mattering. Anger is quicker and louder, and it has one great advantage over all of them. It does not feel like weakness.
So the mind reaches for it. Someone talks over you in a meeting and what arrives first is a flicker of humiliation, gone in half a second, replaced immediately by irritation you could describe in detail for an hour. The irritation is real. It is also the second thing that happened.
Anger is a guard at a door. The question is never why it shouted. The question is what is behind the door.
Why suppressing it does not work
The usual advice is to manage anger: count, breathe, walk away. Those are useful in the moment and they do nothing about the cause, because the cause is not the anger. Pushing it down leaves the guard at the door and simply asks him to be quieter. He will be, for a while, and then something small will happen on a Tuesday and he will not be.
The other failure is the opposite: treating every flare as truth and acting on it. Anger is information about something, but it is a poor guide to what to do, because it is designed for speed rather than accuracy.
The question that opens it
There is a better question than why am I so angry, and it is: what was I feeling in the half second before? Most people can find it if they look. It is usually small, usually old, and usually the thing that would have been unbearable to feel at the time it first happened.
That is what the guard is standing in front of. Not a flaw. Something that was once too much to feel, and got a bodyguard instead.
What to do with it on paper
This work goes badly in conversation and well on a page, because a page does not interrupt, does not get defensive, and does not need you to be fair while you are still hot. Write the incident plainly. Then write the half second before it. Then write what that reminds you of, and keep going back until something lands with a small physical thud.
You will know when you have found it, because the anger loosens on its own. Not because you talked yourself out of it, but because it was never the point.