ok you are pretty good at using mario but you need to use the moves that matter for mario against marth.
Neutral Air - They are good for when marth constantly SH towards you. I use this move for its speed because neutral air can lead to a grab, a tilt, or a Up air
Up Air - Mario can Up air combo marth after 60%. Once you catch him with a Up air, you can usually Forward smash after it.
Back air - when at low percetage, you can pull off two back airs into a Forward smash. this is one of our best kill moves besides Forward smash and down smash which marth's Forward smash can outreach both of those moves.
Fireballs - some Marth will just try to outreach you. fireballs can draw them to you so once they are in your face you will have a better chance since most their moves will not be tippers.
Do not Down B a marth. It will only lead to you getting punished and the punishment is worse then the Down B so its not a good exchange.
Up tilt - this isnt a move that you should do often. The only reason you should use this move is when you Down Throw Marth because you will have enough time to Up tilt then start a Up air combo
the secret to beating Marth as a mario is keep Marth above you at all times. Marth may have reach over mario but when mario is under marth, its a whole new ball game. Not to mention if the Marth is a CC'er because Forward airs are excellent combo starters at almost any damage.
Oh and mindgames is almost a must. if you come at a Marth the same way too many times, chances are you are going to lose
