My advice: Aaron is a bug type user, right? First he uses Yanmega, which is weak aganist rock, it'll take 4X the damge, because, it's a Bug/Flying Dual Type, flying types, ice types, fire types, and electric types are good to use as well. For Bertha, use grass type moves on her Golem, Rhyperior, and, Wishcash for 4X the damage. Hit Gliscor with a strong ice type move, it'll do 4X the damage, and, for Hippowdon, a strong water, grass, or ice type should finish it off in no time. Next up is Flint all of his Pokemon are fire types, so they're all weak aganist water, and, ground. Infernape is the only one that's not weak aganist rock, because, it's a Fire/Fighting Dual Type. Other than that, use powerful ground/water type moves, and, it should be over pretty quick. Lastly, before Cynthia, is Lucian, everyone of his Pokemon, except Bronzong, are weak aganist ghost types, Bronzong is weak aganist fire, so, my advice, use bug, dark, and, ghost type moves for Mr. Mime, Espeon, and Alakazam, flying and ghost type moves for Gallade, and, fire type moves for Bronzong. It should finish him off in no time. Now you have to face Champion Chynthia, for her Spiritomb, use a fighting type that knows the move Foresight, which allows normal and fighting type moves to hit ghost types, for Togekiss and Garchomp, use a strong ice type move, Garchomp'll take 4X the damage from ice type moves, her Lucario is best handled by Fire Blast, Earthquake, and, Focus Blast, which are all powerful moves to the types he's weak against. Roserade should faint after a Fire Blast, or two, and, for her Milotic, you should use grass and electric type moves. Well, I hope this helps you, and you become the Pokemon Champion!!
Yes confidence does help. Training to get higher levels might help even more. Make sure that you are able to defeat the pokemon that you are battling with a good match up. Having Fire, water, grass, ground, dark/ghost, dragon type moves and pokemon(for STAB)should help a lot.
Don't worry! I had trouble to. Just level up your Pokemon and try, try, try again! (If you have another Pokemon game you could also bring your best Pokemon from there over to your Platinum game)(of coarse you will also need another D.S)
my advice will be 2 use the vs seeker at the route before sunnyshore town again and again on the trainers there, the pokemon get pretty high lvl there. then just train and train. (best to fight gyarados trainers higher exp) then try again. you can find more tips on www.seribii.net good luck