I think it's selfish of my friend to not negotiate events for a competition with me.
Why do you think it's selfish of them to not negotiate with you?
When we started this cooperation we agreed that we both will decide events.
I sometimes forget my responsibilities and my promises to people, but your friend has not kept their word and it is wrong for them to do so. How about you report them to your superior, if there may be something that they can do so that you would still keep to compete on the event.
Yeah, I will most likely be needing to talk to my superior. This is a very touchy topic.
It's good that you recognize this. Hopefully if you all get together and talk it out, the problem will be resolved.