Hmm, when I watch it a second time I'll have a lot more to say. For now, I can give you a few pointers.
A) Running up and dash grabbing D3 = great idea. D3s love their ******** grab range, and hence shield grab everything. Just grabbing them gets by this easy especially if they aren't big fans of spot dodging. Grabbing > dancing blade if your opponent is shielding
B) Stop rushing into opponent's that are laying on the ground unless you are going to use counter. Even then, just shield and then punish.
C) Learn to walk.
D) Your LACK of Fsmashes was actually very good. You had them when it counted.
E) Pay more attention when you're using an aerial. I saw so many times when you're Bair would've hit, but you threw Nair, or your Uair would've hit, but you threw Bair, etc.
F) Pretty good use of offensive specials (Dancing Blade/Shield Breaker). Now learn how to use the defensive specials (Dolphin Slash/Counter)
G) Practice your spike. The one time it would've been really feasible to spike, you didn't even try.
H) Learn your spacing. There were a horrible lack of tippers. Smashes are good to use sometime.
I) And of course, like every Marth, you need to tilt more.