Because i think he's gonna hit me.
Hit him instead, or just move away. One thing that a lot of Marth players underestimate is how good just WALKING a tad bit out of range is. It can really add to your game. Maybe watch HRNUT play a few matches, he seems to manage that really well.
I also have some trouble punishing air dodges when i'm juggling
Even as Marth, juggling a good opponent is difficult. You basically have to know what they think you're going to do, and use that against them. Sometimes jumping at them will bait them into air dodge, at which point you can punish with anything you want. I'm a big fan of Short Hopping under them, for a bit, and then just waiting on the ground and Utilting or Usmashing while they try to air dodge above me, expecting me to come up and attack. Nair is your best bet for punishing air dodges, but even then they have to time it wrong, because most characters have air dodges that last longer than Marth's Nair.
The worst thing about Marth, imho, is that after a while he gets easy to predict, so that you have to go out of your way to not be easily read.
Time
Runs
Away