First, tell yourself that you're only human and that your feelings are completely natural.
And that having unresolved feelings doesn't mean that you want to argue with your girlfriend. Having unresolved feelings just means that you have unresolved feelings.
And yes, if you're upset, you should tell her you're still upset. You could try denying it, but what would be the point. Our emotions can manifest themselves in many different ways. And there is no point in denying our emotions to a person who knows us well and who can already read us like a book.
Second, unresolved feelings may not seem perfectly rational either. And by that, I don't mean to deny your feelings, nor do I mean that your feelings are invalid. They're certainly not. I just mean that feelings don't seem to always respond to logic and that they can be hard to decipher on occasions.
For instance, imagine two (almost) identical universes where the same incident occurred. In one universe, the boyfriend gets upset. In the second universe, the boyfriend doesn't get upset. When dealing with emotions, those two seemingly contrary outcomes are perfectly plausible.
What was the initial difference between those two universes do you think? Well, there is an infinite number of possibilities, but if I had to come up with one possible explanation.
I might suggest that the second universe contained a boyfriend that had other potential plans for that same weekend, and that when he heard that the weekend was canceled, that he might have actually been delighted to hear that he could attend to those other plans.
This is actually one of the reasons I said in my earlier answer (to the previous question) that he should focus on himself, his interests, and his hobbies. Depending on himself for his own needs is a way for him to replenish his own personal and emotional energy.
And I know CoffeineConverter interpreted my post to mean that he should "shut himself away", but on the contrary, that couldn't be further from what I was advising him to do. When I said "himself, his interests, and his hobbies", I didn't mean that he should wallow in his self-pity, stay home all weekend, and just play video games or watch TV.
My apology if that's what you understood. What I meant is that he should go out hiking, canoeing, camping, sailing, etc., all by himself or with his friends, and try to enjoy himself even if his original weekend plans with his girlfriend got canceled.
But at the same time, I don't think he should partake in planning the next weekend. He was already emotionally invested in the first weekend. He doesn't need to be emotionally invested in a second weekend outing too.
He also needs to find out if the girlfriend is really interested in going to such a weekend event herself. It could just be that she said 'yes' initially because she felt pressured into it by her boyfriend's enthusiasm.
And last but not least, young people in a couple tend to love taking turns challenging and teasing each other. That's how they have fun. This is why this cancellation could be used as an opportunity instead. Now that the boyfriend got canceled on, he could take that opportunity to challenge the girlfriend to make it up to him.