Practice does help, lol. I wasn't too warmed up myself, so when I saw the Game & Watch I was thinking, "Oh man... I'm screwed."
Game & Watch, Ness and Lucas are easily my worst match-ups for Byleth.
Though, some tips I'd like to give you for future matches are you hover in your Up-B a little longer than you should, and you should use UpSmash in neutral a bit more since it can catch whiffs often due to its invulnerability frames and fast start-up.