In Melee, once you got your opponent off the stage, they're usually dead. In Brawl, usually they're much worse off, but not dead.
It's all about the mind games son, but you can't do no d@mn mindgames with no mind.
You also can't make the right regular choices without the know-how, and I HIGHLY doubt you've mastered using Marth's aerials appropriately, considering how variable the circumstances for using Uair and Bair can be.
Situational knowledge is just as important as numeric knowledge. When you're in that grand finals set, and your opponent is Lucario and about 110 pixels in front of you, and 15 below you, and you're both at last stock, high percentage, you're going to want to know that Aura Sphere will hit you first if you throw ShieldBreaker. Simply knowing that Shieldbreaker has enough range to tipper and finish the match isn't enough.
Get your knowledge up.