You should not only save the matches you've won, but also the ones you've lost, that way you can see where you went wrong and how you could fix the issues you're dealing with at the moment. From the looks of it, you use Palutina. I don't know if she has projectile or not, but if she does, and assuming it's good, as in, won't leave you open, then use that to your advantage. Especially against someone like Bowser. And if you're feeling bad that you lost against a rather slow character, don't. He's a powerhouse, so his strength makes up for his speed. Right now, I'm having trouble facing off against Little Mac because of his speed and strength, and his constant rolling. I was fortunate enough to defeat my opponent, and I saved my replays to see where I was messing up.
Sometimes you've got to tell yourself to stop doing stupid things while going against another player. For example, a few days ago I purposely did a down air on my opponent while coming back from his up-smash against me, and what did he do? He hit me. I did it again, got the same punishment, and thought, "I deserved that. Why am I doing this?" I had to take a different approach. As for spacing, I don't know what Palutina is like, so maybe you can get some help from those parts on the Character Discussions. To put it in simple terms, attack and pull back, but don't make it predictable.
Duuude, I haven't seen you in forever!
And you're right; fundamentals play a very huge part in this game since hitboxes in general are way more specific and rolls are slightly more difficult to punish.
In general, characters don't have as great of an air game (aside from a select few that are able to get away with a bit too much), so there's either a forced approach or an obstacle course of projectiles.
Spacing is very important, but more specifically in the form of positioning, finding the optimal spot to "hang around" rather than pressure with attacks (most characters [at least my favorites] just absolutely cannot land with aerials because of the cooldown, and even pressuring with them is tough considering people don't really favor the air anymore after the airdodge nerf).
If you can hang around a spot and understand what you can intercept with or approach with semi-reliably, you're in a good spot.
And sometimes, matches still take too long, even with all of that good stuff.
This is where the hard reads come in, and generally, it isn't hard to make them since the engine allows for some stupidity.
If I dash towards someone, the optimal strategy (sometimes) is for them to shieldgrab, but I choose to run past them often because I expect a roll; sometimes I get it, sometimes I don't, but if they remain in shield and you recognize this fast enough, chances are that you can do a turnaround grab and basically get a free throw for just dashing past them.
Rolls are good in this game, but don't get locked down in them -- find that optimal range to hang out in until you've got a plan, then roll out and away if things don't pan out.
It sounds simple, but it actually requires a lot of thought and practice.