An act of betrayal of the company. it's hard.

People around you are born with pride and jealousy. This is because they envy people who are recognized and not themselves. Society is about evaluating people, isn't it? But you thought about that chance and chose it. Even if you have regrets about a path you haven't seen yet, you won't know until you move forward.
I wonder if you've betrayed me. All you have to do is properly change jobs within the rules of society. It may be difficult to work in your current environment for half a year, but it will probably move on a path where you can grow. If so, let's move forward. Switch your feelings◎
I'm rooting for you (*^^*)

You seem like a really kind person... 〈They highly praised me and received an enthusiastic invitation to work with people like you.〉 As a result, it looks like “I decided to go ahead and think it would be a learning experience”... but it seems like they have regrets.
First, what is the relationship between company bosses (probably managers) and labor unions?
① There is a confrontational relationship, and is there collective bargaining such as labor-management negotiations?
② Is it a cooperative route, and the union and company are making various decisions while discussing?

In the case of ①; I think you'll be “caught in the middle” and suffer because of your personality. Please honestly talk to a “great person in the labor union” and return them to the company. However, even if you return to the company, you may not be able to “advance in your career” like before.
In the case of ②; try to overcome your current mental suffering and make an effort to “get better” for both the company and the union.

In any case, once you've taken a step, it's probably impossible for it to “get back to normal.” Please be a person who is kinder than ever before, and in a sense ❝ an unsung hero ❞ that makes people around you happy.

I read NB's question. Other than studying, the point of becoming a labor union officer is not written, but it probably means that the fact that I was highly praised and my fervent solicitation paid off. The role of labor unions is to check corporate management that has changed perspectives and return profits to union members.
I became a trade union officer two to three years after I joined the company. There was so much to learn, and I was able to look deep into corporate management at a young age. There were a lot of negotiations, so I learned techniques such as finding a landing point at an early stage and negotiating. I said a lot of things that were deafening for the company, and I think the company executives wanted to smoke.
There are many forms of labor unions. There are professional colleges that abandon their status as employees, black colleges that remain employees and are full-time employees of the union, and labor unions that use time other than work without full-time employees.
Other than professional majors, it is normal that a path back is secured.
After working as an executive for more than ten years, I became a non-union member and got ahead in my career without any handicap in union activities. Rather, since I was young, I was able to remember the name of the company's executive class, so there may have been a lot of lucky aspects.
Also, depending on the company, there are places where becoming a union officer is a common path along the way to success. The biggest merit is that you can see your abilities as a human being, whether good or bad. Labor unions are not a place to think about yourself. It's a place to think about terminal workers. As a result, even if your career advancement is delayed or you don't get ahead, I think there are many things you can gain from that. Once you retire from the company, your relationships there are almost over. What supports me now is a trade union colleague I met when I was young. Some of them have become long-distance parishioners.
However, you must decide for yourself which path you choose. Whether it's a good result or a bad result, please make a decision after being prepared to take care of it yourself.