It is sometimes difficult to grasp when you try so hard, and yet...
That is the problem right there: the "and yet..." And yet you aren't good enough, old enough, pretty or handsome enough, tall enough, smart enough. There are so many times when someone might claim that you are not enough. It isn't right in many cases, but it happens.
Okay, so maybe your two year old sister is not big enough to ride The Torturer at the local theme park; that's understandable. However, when someone makes you feel inferior or takes you for granted, it is a much different situation.
It's just you cannot change the other group of people by force. It would be nice sometimes if it were that easy, but life isn't easy.
You can't change the past, as many people before me have said. You can change how you are in your present and future; so much of it depends on your attitude. That statement does not necessarily mean if you think positively everything will be great all the time. Everything is definitely not great all the time. That's just the way it goes.
However, it sure can make your life a heck of a lot better! As the late and great Randy Pausch said, "We cannot change the cards we are dealt, just how we play play the game."