I think by understanding the feeling you start the process of changing it, but that takes time. What I think is more pressing than changing the feeling is changing how you react to it.
Here you describe your dynamic with your SO
Originally Posted by InfinitePossibility
I know that this seems very minor but I have a very strong emotional reaction to plans changing and it being communicated in a way that seems to dismiss me. I become upset and angry beyond what is reasonable for the situation.
This causes problems between my SO and I - he becomes defensive and angry too. And then we fight.
It's very good that you have noticed your part in that. Have you talked to him about it? You could start a discussion about how you've noticed a pattern of behaviour between you. Try to communicate in a way that brings it across that you take the responsibility for your behaviour, and realise that your reaction is often unreasonable, and that the reason for bringing it up is that maybe he could help you to change it.
I'm not saying it's his responsibility, merely that he is in a position to help you. When both of you are aware of the issue, there are two people who can take notice when it's happening again. Maybe the two of you can together figure out ways to address this. Like, for instance, when he tells you that there has been a change in plans, if he remembers to he could say something like "I know I told you I'm coming home at 5, I'm sorry I know that you don't like to be surprised by these things but I can't make it until 8 because of X". That's just an example, obviously I don't know how he usually lets you know. What I'm looking for is that if he knows you're trying to work on it, maybe he could help you by trying to say something that makes you feel like he is considerate your feelings.
Anyway, the main part isn't that, but the emotions that come up for you and your reaction to them. You write that you feel angry and become upset. How does that manifest itself? How exactly do you act once you become angry and upset? Lashing out, blaming, passive-aggressive? Whatever it is, try to stop yourself from doing that. That may not be easy, and will not probably always work, but you might be able to do that at times if you are conscious about it.
The hard part in that is that anger will make you feel that your response is completely justified in that situation because you feel that your SO was inconsiderate, and therefore if you refrain from "punishing" your SO, he will get away with it. But you can make a deal with yourself: you do your best not to react in your usual manner even though you feel that your anger is justified. Rather, when you feel angry and upset, you sit with it (and go over in your head why your SO is the meanest and most inconsiderate person in the world, as the anger makes you feel) and remind yourself that you don't have to let him know what you feel right away (by e.g. lashing out) but you can wait until you're calm. Then after you've calmed down, if you still feel that he acted inconsiderately towards you, you can tell him how his actions made you feel. And if you don't, then it was good that you managed to refrain from "punishing" him.
Now, if you fail in your attempt to control your reaction to the anger, you can still do something to rectify the situation as soon as you catch yourself in it. You can force yourself, at that moment, to stop whatever it is you do when you're angry, and make yourself apologise. Even when that feels like the last thing you want to do, it isn't impossible to utter the words "I'm feeling really strong emotions right now and I'm aware that even though I feel angry it's not your fault. I'm sorry that I'm taking it out on you.". By doing that, you show him that he doesn't need to be defensive, which takes the edge off his anger. That way you can avoid the fight, or stop it in its tracks. And that positive result is something you can try to remember when you feel stubborn and angry and don't want to admit you might be overreacting.
Again, your SO can help you with this step. If he is aware of the issue, he can note when he is triggered into defensive mode by your anger, and at that point rather than escalating he can ask you "Are you sure your acting reasonably?". Now, that may be hard for him to do. But if he does it, it is again your responsibility to swallow your pride and admit that you may be overreacting. And then again you won't get into a fight.
Hope this is helpful.