anger and disappointments

Anger
I know what's anger. I've been feeling angry since forever.
I'm always angry towards the people around me, towards the government, towards the computer for not having a stable line, towards the screwed up KTM and more so I'll admit that I'm a very bitter person. But lately, I've been exposed to another strong negative emotion-

Disappointment
I'm disappointed with my family members, and now hatred, anger and disappointment towards a person unworthy for me to call friend.

I despise people who break promises and despise people who are not punctual all the time.

This unworthy person's story is very straight forward.
A is flying off to UK this coming Sunday --> A, B and C on Monday made plans to have a farewell dinner on the coming Friday for A --> On Friday, A asked if they can meet at KL instead of Kepong and B was OK BUT C replied to A that she has something to do so won't be going.

I know that
a) C don't want to take the trouble to come KL (she said yes to Kepong because C stays at Kepong BUT A stays at Puchong which is very far from Kepong so now to make it fair, KL is the best place and will there be another chance to meet A??)
or
b) she was asked by the ballet teacher to replace a night class (not acceptable because we made this plan earlier this week and there are other students who can replace the class)
or
c) she is just stingy and don't want to waste money (I say so because she won't reply or call you because to her it's a waste of money and hence the stingy personality).

Anyway, I'm even more angry because A suggested that maybe C suddenly have something to do and so my question now is, why didn't C have the courtesy to actually inform us that she's "suddenly" busy?? And thus I think option (a), (b) and (c) can explain her not coming for the dinner at KL.

Well, this is not the first time I'm feeling so much anger and disappointment towards her but I'm going to make it the last since I will try not to talk to her so much because

if you don't care about that person, you won't feel angry, disappointed or hate

I know I'm being mean and cranky but trust me, this is mild compared to what I'm going through.