Hello @macchiata and welcome to the community
I am reading and trying to put myself in your shoes and I see how strong the situation is especially from an emotional point of view. I have known of several cases of parents changing for the reason of a new relationship and I am sorry for what you are going through.
On the other hand, I think you are a very good person, to approach your father and stepmother after all, to take a plane and travel so far to visit them and receive bad words from the other person, I think is admirable, other people would just look away, but you keep your conscience at peace as you are doing the right thing.
A big hug
I just have to do what I have to do. There's part of me that just wants to help because I simply like extending my hands to someone who needs it. It wasn't easy to do all of that but to make peace and to leave me no regrets one day, I think I did the right thing.