I was just as bad when I started. It was my first stratagy rpg ever, and it didn't help that most of the game (and all fire emblem games) are pure luck.
Oh... you can restart from the chapter save as many times as you want, so you can try for a different chapter. You would not belive the amount of times I've done this. Though if you want to keep the save youre playing, you can just skip doing that.
Yeah, I'm kinda regretting killing of Frey in the prologue (I'm trying to get to all the gaiden chapters), because Cain was horrible after the first few chapters. He couldn't double ANYTHING.