How can I get rid of the pain of flashbacks?　

Flashbacks are caused by your brain.
It would be difficult to put pepper on your nose and not let your nose sneeze.
The brain is like that too. It responds to stimuli.
So it's hard to keep flashbacks from happening.
When a flashback occurs, there is no choice but to return to a normal state of mind as soon as possible.
Cut your delusions quickly.
To that end, I think it's a good idea to practice cutting out delusions on a regular basis.
In meditation, when consciousness deviates from a specific object, consciousness is quickly returned to the original object.
Why don't you try something like that?
In the world of Nembutsu, it is said that even if delusional distractions occur, it is good to keep happening, but since delusional distractions cannot be continuously continued while actually continuing nembutsu, as a result, nembutsu slashes delusional distractions away.
Nembutsu is sometimes called a sharp sword.

　I think flashbacks are great, Mr. Hokutosei. I also suddenly remember my old memories of being bullied and the fact that I didn't share my life and said, “I hate it!!!” It will be. But I don't care about it forever. Is that because you have been doing zazen meditation during ascetic practices at Motoyama? I was told to leave them alone even if distractions or delusions come to mind during zazen meditation. That's because it's something that will disappear someday... The result of repeating it. My heart says, “It's not going to happen, OK?” It became. Flashbacks show some regret, embarrassment, frustration, etc., but don't worry about it. Furthermore, I began to want to do something about people in the same situation as myself.
Thus, when dealing with people, I try not to forget to stay close to them as much as possible. When the other person feels uncomfortable because I don't have enough words, I apologize, reflect on it, and try to make use of it next time. That's all an eagle can do.
I'm often told not to do anything superfluous. But is that really OK? I only live once, so I want to get involved with as many different people as possible.

What can I do to prevent flashbacks from happening?
That being said, it's not about preventing it from happening, it's about thinking that it's OK to happen.

Thinking it's wrong = being caught up in the past
So that's it.

Not being caught means staying the way it is. Then, while thinking it's okay to wake up, when my thoughts have gone astray in the past, I try to look at the present. Observe your current state of affairs.

What you see, the sound, the scent, the feeling of your own body... please feel your present.

When I'm feeling, there's no past. Now is the only time.

Every day, tens to tens of thousands of thoughts come out of the human mind.
Many people won't be aware of this unless they measure their brain waves.
You probably don't care about those thoughts.
Among them, the only thing that bothers you is thoughts that are not convenient for you, things you don't want to think about, or feelings you don't like, and that is why you hold on to that and evaluate and fight back.
In other words, at the time of occurrence, there are no good or bad thoughts.
Please spend your time keeping your distance the next time you think it's going to be a flashback.
It's a transient thing, so it will always go away.
The best thing is to receive guidance on Zen or meditation under a monk who has gained proper enlightenment.
If you don't influence yourself from your thoughts, your heart won't decrease, increase, or get hurt just because flashbacks come out.
Try spending the following time when you get flashbacks in the future.
➀ A thought came out. Even if I say it came out, it was fully automatic at the time it occurred, so it just came out.
❷ Normally, I expand my thoughts by clicking on a PC file.
❸ As a result, I suffer from thinking bad things.
Therefore,
② Right after my thoughts come up, I spend my time watching fireworks as if keeping a distance from those thoughts.
or
② It's about pouring awareness into facts you don't think about.
If you choose to keep thinking, your mind will lose your mind.
If you cut the thought mode and look at the facts, your thoughts will ❝ run out ❞.