It feels to me like you're not able to keep up with that Fox. Partially because that Fox is pretty fast, but also because you're committing to a lot of laggy things. Like wavedashing, for one, I feel like you're incorporating way too much into your movement game, especially since it tends to be rather compact, so it's fifteen frames of closed-space lag, which is going to hurt you.
Other things are like... your choice to use the Dancing Blade (which does not seem to be paying off at all) and to continue the combo even when it's obvious you're not going to hit him. You also seem to like to platform dash to the center of the stage, which is fine, but, you're doing it in situations that don't call for it, so you're just putting yourself above Fox on an extremely predictable course.
There're also times where I'm really just wondering what the hell you were thinking... dropping straight down on Fox on the recovery and dying to a usmash... utilting when he's approaching you on the ground... jumping away from the jab instead of fairing... spot dodging after he dash attacks your shield instead of grabbing.
It seems like you're panicking and trying to outspeed your opponent. Well, you can't, because you're Marth and he's Fox. Properly played, he'll always be faster than you. It just seems like you're jamming buttons and button combinations, trying to force Marth to go faster, trying to force your game to go faster and, as a result, you're missing simple edge guards and mis-spacing. Don't get trapped in your opponent's rhythm. You gotta take control of the match, because that's how Marth wins.
Unless you're Mew2king and your Marth somehow is Fox-like in speed. I'll get back to you on that if this is the case.