I'm mad at my friend because he cancelled important plans we had together.
Why do you think he cancelled your plans?  Do you think you could find out what his reasons were?
I think he wants to hang out with his unworthy friends rather than me, and he only wants to hang out when it's convenient for him.
Your friend should have been more considerate of your feelings, and known that his decision would have affected you negatively.  You should probably be willing to forgive him, if this only happens rarely, but if he makes a habit of it, you might be better off finding another friend to spend most of your time with.
No, he will always be a loser for being so careless.
Well, I know you must be hurt. But, you should also hear him out and find out his reasoning. It's better to forgiven than be bitter.