You have a few bad habits you should break immediately.
1) Don't approach with forward air. He can just shield grab you.
2) Don't roll back when you land a move, there's no point to it.
3) Never roll into your opponent. Ever...ever...bad idea, especially if he begins spamming down smash.
You should learn dash dancing and crouch canceling. It's so easy that I'll explain it now!
Dash dance - the process of dashing toward your opponent then dashing away then dashing toward your opponent. Makes it so that they don't know when you're going to actually attack them. You can grab or dash attack or stop or continue or whatever.
Crouch cancel - Holding down to crouch when you're about to be hit at
low percents. The knock back from the attack will be reduced when it hits you and if the attack isn't a really strong one you're not knocked down and can act almost immediately.
Things to try -
1) Walk, don't run. Then use forward tilt. This is good when you're starting out and is more reliable than forward smash.
2) Go off the edge and hit him. Just jump once, back air, jump back to the ledge. You can do this with any aerial though recovering will be different.
3) Use tilts more (guess this is 2.5).
Side notes -
You can't chain grab him.
Try to prevent your opponent from getting to the center of the stage and keep them as far away from it as possible.
Guides - Everything you need to know, want to know, will forget, and will rediscover here
Marth specific -
http://www.meleeitonme.com/sharpening-your-sword-tais-marth-guide/
Competitive stuff -
https://docs.google.com/document/d/1_6o5NHvI9P8-qHp2M8w1gDzL_-JNonEE81Mh4ulY1PY/edit#