It sounds as though you have fallen into the trap that most people who despise the game have, and expect that mashing the X button and spamming attack cards is how you play the game.
Step 1: Understand that this is not the original Kingdom Hearts game, so any bad habits that you may have picked up while playing (swinging the Keyblade blindly and relentlessly) should be dropped.
Step 2: Take some time to understand how the card battle system works; it isn't complicated. Sleights and 0 Card Breaks are the key to all boss battles.
As I said, the people who struggle with Chain of Memories tend to be the ones who try to play it like the other games which require absolutely no thought other than moving in the general direction of the enemy and attack, attack, attack. If you don't like to be challenged and prefer more rudimentary games I suggest that you just start over and play on Easy.