Will comment on vs Rosalina.
First mistake: you probably shouldn't have gone to Battlefield. Especially not against someone like Rosalina who has really good coverage above her.
Your next critical mistake was you went for a fireball rather than getting to the ground. This put you in a really bad position as M2K was ready to powershield this and get under you to put pressure. You also messed up jumping out of shield on the platform as you tried to escape.
You needed to pay better attention to M2K's movements when you were recovering. Take 0:48 for instance. When M2K jumped, your best option at this point was to quickly jump and go for a higher recovery to intercept him with Up-B. 1:57 you wasted your midair jump for no reason to recover high while M2K was still sitting on the stage, and got gimped for it. Things like that you just can't be careless about.
Your approach is a bit telegraphed and too aggressive, which M2K was ready to punish most of the time. To give an obvious example of a situation you could have played very differently on reaction, at around 1:08, you were approaching with fullhop B-air, but you had time to observe M2K was retreating with a short hop facing backwards. You could for instance B reverse Fireball completely safely, which would likely punish M2K's landing for some chip damage. Just in general, I felt like you were rushing in too much, and M2K was generally ready to respond to this. He's historically a defensive player, and rushing in predictably is what he wants you to do.
Around 1:48 was an ideal position to attempt F-air (M2K's only option to survive this would be to airdodge, but he doesn't have the same reaction time as other professional level players). Around 1:51, why did you get up from the ledge? Even with his jump, M2K couldn't really reach you with any aerials, and staying on the ledge could have given you an opportunity to at least try to N-air M2K.
Other random thing I felt in this match is in XeroXen's words, use more Up-B. The way M2K spaced in general with aerials left him open to being punished by Up-B in a fair number of situations, and on BF especially this is something you want to try to abuse here and there. By forcing M2K to play more of a grounded game, you would have more opportunities to properly get in with either D-air or B-throw to secure some control in the matchup.
Now game 2, I don't completely agree with landing with D-air, but it worked so props. 3:31, you really just got cheesed because you just seemed to not be aware of that shenanigan. You really just gotta pay attention to movements, and know that sometimes when recovering you gotta react with Up-B to anti-air someone when they move in position. Especially against Rosalina who is floaty, she's less likely to punish you on hit for landing Up-B at low percents. As a player, I also hate spotdodging, and given that U-smash read M2K did around 5:37, I suggest you ask yourself if you really had any reason to consider spotdodging (obviously to avoid grab, but considering how much damage you had, was it really a wise choice in the first place?). And you lost your last stock with an apparently panicked fullhop (fullhops aren't bad vs Rosa, but I felt like for much of the set you made fairly poor use of them).
Don't have much to comment on game 3 given you still lost to offstage shenanigans. Anyhow in summation, you really have to play your recovery better and be more patient in neutral.