Concerning the match vs. marth:
You missed a lot of SHFFLs and got punished for it. SHFFL'd aerial ~> gentlemannings will **** him if he tries to shield grab, and you can roll away if he shields the gentleman. Nair to grab if he's at lower percents, and nair to nair/u-air (hopefully ending in a knee) at higher percents.
Marth was just waiting for you to poorly approach and out-prioritize you. Dash dance, empty shorthop to grab, and wavedash back (not too much), to bait him. Also, dair a bit more (but don't spam it). Depending on the percent, it can lead to his timely death.
In short, work on tech-skill, know what to do at what percent, and be less predictable. Keep up the good work.
P.S. If you're going to go to the trouble of L-cancelling a killing knee, why not throw in an immediate taunt to lower his confidence? ^^