Boxing is definitely more mental than physical.
You can always train yourself to be in better shape, the option is there, you just need the proper motivation.
Mental aspect, not so much. You cant really train yourself to be mentally prepared. You CAN gain confidence over time from training, etc. Some people are more determined, disciplined, and hungrier than others. Thats why boxing was originally a sport for the middle to lower class people. It was a way for them to escape poverty and find success or fame. Thats why so many boxers come from poor backgrounds, especially the good ones, they are mentally tough from growing up having nothing. They are hungry for success because it doesnt come easy.
You have to mentally have the drive to succeed in boxing. It isnt a team sport, its only you and the other guy in the ring, only one can win (usually).