I'll happily look over your stuff if you PM me a link (sorry, didn't see this 5 hours ago when you posted it). Rosalina is top tier so switching off her is probably not the solution to your woes. She's really complicated so you'll have to do some work to make her work, but she's totally worth it and if you're starting out you have some work to put in anyway so if you like her... why not use her?
EDIT:
I don't want to be the only one to critique; this match is actually a pretty good one to look at IMO and other players might have other useful perspectives. This player isn't even really bad; I expected a lot worse from his downplaying of self in the wording of the topic (though be warned the commentary is awful, but that's not the player's fault):
Here's what I'm thinking:
Early on...
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Not respecting Fortress (kinda charge into it a lot; it beats just about everything but isn't safe so you have to bait it out/block it more)
Misspacing a few hits that would have made your early lead huge
Find better landings against Bowser's usmash. If you time your dair you should be able to hit him out of that; that carries risks but you can't afford to get usmashed every time you need to land from a juggle situation.
When he makes a huge mistake (like whiffing Bowser Bomb or using Fire Breath in the wrong direction), punish with fsmash not a grab.
By and large you played game one pretty okay. You could have won more decisively, but you did most stuff right and won as a result. You're obviously feeling very confident after this game and it seems very likely you're going to decisively win the set.
Game 2...
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You go in so hard with smash after smash. It pays off for a while, but that huge risk playstyle costs you against a heavy hitter like Bowser. Jab and tilt more!
You take a lot of damage getting desperate on 2nd stock, but honestly, that kill was pretty good.
By and large you just kinda went for big stuff the entirety of game two, like your solid win in game one made you overconfident. It got you an early lead and an early stock loss, and then it resulted in you losing a ton of health by (pretty predictably) just going in hard when behind. Especially against a character like Bowser, you have to play more respectfully, disrupting with jabs and tilts to throw him off and stay safe to set up for your bigger moves and bolder actions.
Game 3...
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Smashville is not a counterpick. I dunno the stage rules but going back to the stage you just lost on is more often a bad idea than a good one.
You overcorrect and play way too scared early on instead of way too aggressively like in game 2. When Luma is going out like that pressuring (which is good, wish you did it more), you need to be ready to follow up with Rosalina if they punish the Luma like Bowser did. You need to try to maintain some kind of stage control as well so, when Luma respawns, you'll have a chance to use it.
Stop using airdodge to escape tumble. If you must throw out a move to do it, nair is less laggy.
When things get tense, you stop playing for any kind of control and start airdodging low to the ground/going for bigger attacks a lot as a freak out. Bowser is very good at punishing that which ended you.
So, if I were to summarize the biggest reasons you lost the set, it would come down to this. You seemed very uncomfortable in the match-up and didn't really adjust your risk taking to Bowser level rewards and challenged his moves with the wrong stuff instead of blocking some stuff or using different moves against other stuff. Like a lot of the "just go for it" stuff would make more sense if your opponent hit as hard as Sheik and maybe you can accept losing out some more, but it's Bowser so you can't really take a lot of punishes from when you go in too hard with slow stuff. You did a fairly poor job of controlling space; you were constantly backing off, struggling to land, and otherwise being bullied by Bowser as opposed to you being the one bullying Bowser. This mostly stems from your non-use of jabs and tilts as well as your rare use of moving Luma around with jab/ftilt/dtilt and actual zero uses of Star Bits. Not controlling the stage gave him time to find bad situations to hit you with early kill moves, made it hard for you to keep Luma alive since you so frequently had your back to the ledge, and made it just harder for you to land a lot of hits. You generally lacked confidence/control when falling toward him as well which is a more subtle game that is hard to coach about; just get more comfortable playing with your air control/poking with your aerials at the right times/reading their movement to see what they're going to try to do and you should get hit less in those situations.
On the positive side though, you avoided any notably bad habits other than airdodging low to the ground (old Brawl habit?), you used Rosalina's follow up game pretty solidly, you got a lot of really good grab reads (could probably pummel more but that's small potatoes), and other than significantly underusing jab and not using tilts either you actually did show a pretty good command of Rosalina's neutral toolset which is no small thing on a character this complex. Stop beating yourself up and have some confidence; your Rosalina is more good than bad. If anything the way your emotions showed so much in your play is worrying, taking big risks when desperate or confident and going turtle mode when feeling down over a loss. The Bowser kinda did the same stuff all the time, but a lot of opponents would have found you easy to read for that.
I don't think this loss should be a "I need to drop my character" moment. You didn't lose by much, and you seemed to have better fundamentals than your opponent. You just didn't know the match-up and needed to clean up your neutral to have more control (and play with more consistent confidence!), and in the absence of that, how hard Bowser hit very narrowly won him the day. These are fixable with relatively modest effort, and you played that Bowser close enough that you should be able to go back the next event and beat him. In the long run your top tier main and the stuff you are doing right should pay off for you handsomely; even though you were the loser, I'd be much more concerned about winning long term if I were the Bowser here.
