I saw that post but didn't have much of a response to give. I still can't do so now because I'm on my phone, but I advise coming up with a list of the main points you have trouble with regarding Rosalina. Post that here, and from there, people should be able to work something out for you through discussion!
Well, I have a bunch of things that I'm having trouble with. As I said, I've been a DK main ever since the N64 smash so I'm kind of used to playing offensively, going for grabs, blasting them away with strong normals, getting in their face to kill them with my giant punch, that sort of thing. When I trying playing that way with Rosalina I get torn to pieces, so it seems I have quite a lot of habits to unlearn.
Here's just a few of the things I struggle with:
1. The utility of her moves.
Rosalina has very strange moves. Her down+B is only useful against characters with projectiles, her side+B and neutral+B require her to have her luma, and her up+B seems to be strictly a recovery move that doesn't do any damage. On top of that I have trouble with her normal attacks, especially her aerials. I don't understand the utility of her nair, since it seems to attack backwards and then move forward? Nor do I know her best moves to go in for the kill once the enemy has high damage. On the ground her smash attacks are incredibly difficult to land, and she doesn't seem to have any killing moves in the air. Her up air and down air fire this bizarre little slow-moving ring projectile that doesn't seem too useful for anything
2. How to use the luma.
I understand that the luma is meant to help her zone her opponents out but I just don't get how it's supposed to be used or controlled. If I shoot it too far away that leaves a giant gap where my opponent can get between me and my luma and begin a rush towards me before I'm able to get my bearings or call my luma back to me. I don't know how to reliably land the luma shot or the purpose of the star bit spray. I've heard people say the luma's good for edgeguarding but I don't know how to do that with the luma either. They get on the ledge and my luma's not able to do anything. I particularly struggle against the Fire Emblem characters as they're able to rush me down to their heart's content and their neutral B moves can kill my luma in one hit.
3. What to do against zoners/rushers.
Despite my lack of understanding about Rosalina I can do a fine enough job against slow or even mid-speed characters, but I see she has a great deal of trouble keeping out fast characters like Little Mac, ZSS and Sonic. Not only that but she seems to struggle against people who can lame her out, like Sheik; if Sheik wants to keep me away, I have no idea how to get in on her.
4. Rosalina's general gameplan.
Before I can start getting into the bulk of training with her I at least need an idea of what to do with her. Currently I have no clue what her gameplan is supposed to be against all characters, let alone the specific cases that I listed above. Nothing about her feels intuitive and I think I only manage to get by via the skin of my teeth instead of showing proficient gameplay with her. I need basic idea of what she wants to do in general which I have yet to grasp.