Lets be clear, what you have described is more of a problem within yourself as than it is anything the opponent is doing wrong. If the Marth is able to beat you by mindlessly forward smashing, he did exactly what he should have done. You'll run into these types of Foxes and Falcos too, but they all do it for the same reason. Players that mindless rush them will consistently run into their attacks, so that is the most rewarding strategy at their level. You've become accustom to playing against more cerebral players, but you still have to be aware of how to deal with players that haven't mastered their movement yet. Against a fsmash heavy Marth, WD OoS grab is guaranteed as long as it wasn't a tipper (if i'm not mistaken, shield stun lasts too long with tippers). Just imagine a bubble around Marth that is the extent of his sword's reach, and DO NOT enter that bubble as long as he has fsmash available to him. Pretty much everything in Melee has a rock-paper-scissors aspect to it, you just need to learn what your options are and what beats what your opponent is doing. Shield grab dash attacks, spot dodge if he's fishing for grabs, grab him if he's sitting in shield a lot. Just notice what your opponent is doing to get in on you and focus on countering it, you can't always keep one strategy throughout the entire set.