You have my wholehearted sympathy OP. Anyone who works with the general public will come across these arseholes and they really can drag everything down.
I had some thoughts about your story and your reflection. You said you wished you had never given the cup of tea in the first place. But what struck me was that if the original person who asked was perfectly nice about it, then you shouldn’t regret doing that extra little thing and getting the tea. I know if I’d been the recipient, I’d have been very grateful. Then it would have been a positive experience.
The moment when it went wrong was not with you getting the tea, it was when the recipient began to be an arsehole and you continued to try to engage as if they hadn’t. Your regret should perhaps centre around trying to assist after that point.
Try to think through the episode again and imagine it ending differently. Your response to the first complaint (stating they didn’t have to drink all the tea) was reasonable, despite the fact that the person was already showing signs of being ungracious. When they suggested they might spill it, what might you have done instead of what you did? Perhaps you could have said simply, “Well leave it then, if you don’t want it,” and walked away. Or made the offer that if they didn’t want it, you could take it away again. What ending would have left you feeling satisfied with your own side of the exchange, even if you still accepted they were rude and unpleasant?
Work out exactly where it went wrong and at what point you feel your own responses begin to make you feel bad. I’d warrant it was at the point when it became obvious beyond doubt that this person was a troublemaker and you continued to engage.
And think about the feelings you felt. What you need to do is learn to listen for that quiet voice in your head that is telling you that something is wrong. Currently you can barely hear it and instantly overrule it as you have been taught to be polite.
If you begin to listen for that voice, you may start to hear it. You will be rusty and it will take courage to listen and act. The first time you do will be terrifying and you might get it wrong because you lack practice. But the only way to learn assertiveness is to practice it. And when you begin to listen to that voice and respect what it is telling you, only then will you begin to find your self-respect. And then you will see that the shitty experience you had, that was not remotely your fault, was not something to feel guilty over or regret, but was a life experience you can learn from.
That you are still agonising over this is a clear sign it isn’t resolved. Continuing to put yourself through pain over it is counterproductive. Stop beating yourself up. You did a nice thing. Not your fault the other person was an arsehole.
I still struggle with this stuff, especially with certain people, but every time I do get it right, it gives me more confidence that I can and that’s a great feeling.
Good luck.🍀