Yes, it's her first relationship, and I understand that she's still learning. But they've been together for over a year now. At some point, there should be noticeable growth. It feels like there's been little to no improvement, and she shouldn't still be at "level 1" after all this time.
Also, when she said, "Just because there's love doesn't mean everything will work out," it honestly sounded like she's already giving up. Why say something like that if you're supposed to be trying to make the relationship work? It comes across as if she's silently quitting instead of putting in the effort to improve.
“They been together for over a year there should be growth” yet she hidden her self since of age of 7 to 17 years old. You really think she can change that much in over a year? Just like back in high school when she had her song exposed she had a panic attack but over came it with aya support. She’s is changing little by little. That she was able to wear what she want bc of her friends support. When she said “just bc there is love doesn’t mean it will work out” more or so to her passion not fully her relationship. But once again music is mitsuki life and she felt like she just lost it bc of how those girls treated her in that band which is why she can’t tell aya bc she scared she will leave her if they can’t connect about music anymore. But they will have the talk soon. They will work it out. Idk if this is your first time reading this but the full manga books been out. So just be patient with mitsuki she’s at the point where she understand she can’t keep avoiding it. The little brother will help her see that and so on from aya.
I understand that she's been dealing with years of trauma and insecurity, and I'm not expecting her to completely change in just one year. Healing isn't linear, and I get that she's making progress little by little.
It's that after more than a year, I'd expect to see more initiative from her. Growth doesn't mean becoming a completely different person, it means actively trying to communicate, face problems, or take small steps instead of constantly avoiding them.
As for the line, "Just because there's love doesn't mean everything will work out," I know you said it was mostly about her passion for music. But the way it was written also naturally applies to her relationship, especially since her fear of losing music is tied to her fear of losing Aya. That's why it came across as if she'd already accepted failure instead of believing they could work through it together.
And yes, I know she'll eventually grow because the manga is already finished. I'm talking about how her actions feel at this point in the story. I can appreciate her character development while still thinking the pacing is frustrating.

Mitsuki girl, why would u say that?