So if mental toughness = positivity in all circumstances, then we have to answer the question "how to be more positive?" if we want to be more mentally tough.
People are positive mainly because they possess hope. Okay...another definition, so what is hope? Hope is ... "desire or what you want."
So lets go back to the mental toughness. Mentally tough people are those who want something bad enough that they have decided they won't stop until they get it. So to not press on, to not persevere, is not an option. Therefore positive is the only thing they know to be. They aren't thinking about quitting, they aren't thinking about stopping, so a negative outlook just simply doesn't have a place in their lives.
Read the short paragraph above again and this time think about just how very important knowing what you want is to this process. Deciding what it is we want is by far the most important step to becoming mentally tough and also is the one that is often the most overlooked.