You just need to play more against marth to get a feel for the match up and eventually screw ups will decrease. The first marth I played at a tourny, consistently was 3 or 2 stocking me, played him like 7 times in friendlies. Due to miscalculating spacing, getting grabbed, not DI ing correctly, not DD correctly (by this I mean i just DD for no reason off the to side and then run in for aerial and then getting shield grabbed or faired).
But at the next tourny I was winning our matches at a 2 to 1 ratio and I agree that stage is a big factor. I only lost on yoshi and battlefield.
It's good to use uthrow to knee as well. Most will dthrow a marth and so a marth when grabbed will DI away but if you mix in uthrow and they DI away it leads to knee. It works from 65-90 ish or something like that. I usually do it at 80 cause I have an option of dthrow to knee or uthrow to knee.
Know how to combo out of throws, pseudo chainthrow with dthrow, and throw to knee combos and it's one of falcon more fun matches.
Also learn lightshielding a marths upb. It leads to instant edgehog or ledgehopped reverse knee, stomp to knee, get back on stage> grab> combo him to the opposite edge... choices are endless.
Lightshield is roll to edge while holding z and down and away with joystick from the center of the stage then letting go of everything when the marth hits your shield.