Why did Peter forget that he would die an ignominious death?
Peter forgot because he was completely focused on the hopeful prospect that Israel would repent, believe in Jesus, and thereby see the Messiah return and the kingdom restored. The context notes people readily remember good things but quickly forget bad things, and Peter was enraptured by that expectation.
Les explains that this is a common human tendency: we can forget bad things if we want to, and it is easy to remember the good while quickly forgetting the bad. At this point Peter had totally forgotten Jesus' earlier indication in John's Gospel that he would suffer an ignominious death. Instead, Peter was totally enraptured with the possibility that if Israel would repent and believe who Jesus was, then their Messiah would return and they could yet have the kingdom. He was in full agreement with the Old Testament expectation that heaven must receive Christ until the times of restitution, when the curse would be lifted, the tribulation would accomplish its purpose, Satan would be defeated, and the earth would be regenerated for the kingdom reign.