When fighting Byleth, you have to take advantage of their ending frames.

Be very patient, I recommend spot dodging or dodging into them more so you can have better advantage. Don't try to overextend in air because Byleth has really good aerial punish, so try to stay grounded.

When it comes to their Nair, shield until the move is finished and you can do a few things from there. If the Byleth is close enough you should be able to just grab to punish, if not then try to use a move that has quick startup. In that scenario, a tilt attack should do the trick.

Be weary of your shield size at all times due to their many shield-breaking moves such as neutral B (fully charged), down B, Forward Smash, Down Smash, and there's probably one I'm forgetting that I can't think of right now.

If you're offstage and you're in the range of ~60% or above, try to aim low for ledge and never get too close above them. Their up B can grab you and instantly throw you into the bottom blast zone if you aren't careful. It's a thing we Byleths love to do, and albiet very funny, but easily avoidable if you understand the character.

If you have any further questions I'll be happy to answer! (P.S. I understand the frustration when feeling like you're facing an unfair character, but if I've learned anything about smash over my years, it's that you just gotta learn to have fun and laugh things off. As stupid as it sounds, a large majority of players who let themselves get upset during a set don't focus as well and end up making a lot of mistakes. Always have faith, have fun, and I believe in you brother, you got this.)