You know I alway wondered why Marinette kept Cat Noir on the sidelines all of season 4 and I think it finally makes sense to me...
It's not a question of trust but stress.
I think stress has been pilling on Marinette's shoulder this whole time, but especially since becoming the guardian. The problem is even though she trusts Cat Noir with all her heart... she also doesn't. Let me explain.
The order will be wrong but here are some of her reasons I could think of:
The Whole Cat Blanc Thing Happened: Which may constantly make her be on guard subconsciously because she thinks "if he finds out who I am / if I break his heart again the world will end"
Being a Guardian: Master Fu essentially isolated her with all the secrets she had to keep and every time she asked to share with Cat Noir she was told that no, she can't. He made her build the habit to keep things secret from him because he can't be trusted. Not that she doesn't trust him of course but in her mind Master Fu didn't or he always thought it was best he didn't know for now. It became a bad habit and now that he left, she is left all alone to bear the weight. Because she is a guardian, she is Ladybug therefore she can't be crushed. Not because she is strong, because she can't afford to be crushed there is too much at stake... That's the mentality Master Fu left her with.
Finding out Cat Noir's Identity Ended The World: What she is left knowing after Ephemeral is that she found out his identity and something BAD happened. Bad enough that a Kwami had to use their powers without a holder (Which is to never be done because of the consequences) to repair the damages. That's how desperate the situation must've been.
Also in Ephemeral: Master Su-Han basically shows up and has a hissy fit about how you can't let the black cat miraculous holder on the loose because if we don't know his identity, then how can we know if he an be trusted?
She is given constant feedback telling her that she can't trust Cat Noir or that she can't rely on him + all the stress she probably gets having someone at her side that she trusts yes, but if she does one small mistake concerning him then the world explodes. She can't fully relax with him around anymore she has to be on guard. He probably looks like a ticking time bomb at this point. She worries over hurting his feelings, keeping secrets from him, finding out his identity (Which is something he is always pushing for) and when she's fighting she has to hope he won't be too late to help her. None of this is Adrien's fault btw, he has done no wrong. This is to show things look from Marinette's point of view.
So when she tells Alya her identity and the world doesn't immediately implodes? A little bit of weight is lifted off her shoulders. Although she carries a lot still. So what does she do? She surrounds herself with pillars, chooses heroes she knows and trusts and relies on them a little, and then a bit more, she is finally not buried in pressure that almost crushed her and since she doesn't have to worry as much with them she unconsciously relies on them more then Cat Noir not because he can't be trusted, not because she likes them more then him. But just coz it's is easier and it was an unconscious decision that happened over time.
This is how much easier it is in her mind, probably:
There Is No Guilt: She doesn't have to tell them things and they don't expect her too because she is their leader, not their equal. She made it clear from the beginning that there are here to help but can't keep their miraculous but to them that's a gift by itself so no pressure there.
She Has Alya: She has someone she can tell all her secrets too and bonus, she actually knows that she can 100% be trusted. Alya is her backup always a call away and. (I also think that's why she looks the other way when Alya messes up, because she DESPERATELY needs her support)
Cat Noir Can Relax: Although this isn't the case and she doesn't realise how much it's hurting him to be put aside. In her mind if she could take a break, she would, but she can't so at least she can lighten up the weight on her kitty. She doesn't know about his trauma and his need to be acknowledged, it's not either of their faults but without this information she can't possibly see why this would hurt his feelings.
She Can Actually Reach Them: Instead of fighting monsters by herself waiting and hoping for him to show up. (and sometimes he can't and that's not his fault) She can instead get the help she needs right away and have a whole team to support and back her up if anything goes wrong.
She doesn't rely on him as much because it's just easier that way. Less Stressful. That's it.
She does this so she can finally breathe, at last.
The pressure finally stoped dripping.
She finally mended the cracks in her walls.
Until she fully cracked and broke all at once in Strike Back.
All her pillars? GONE. Alya? Can no longer be relied on. All alone and shattered.
Then Hawkmoth threatens to make the whole world turn against her and she’s lost EVERYTHING. Cat Noir shows up and tells her that no she hasn’t lost him.
She gives him all the reasons he should also give up on her too she’s lost the miraculous, she’s a terrible guardian she tries to turn him against her coz she doesn’t believe in her self anymore she can’t do this, but most of all she doesn’t DESERVE him.
She lied, she tried to control everything she pushed him away when all he wanted to do was give her a hand surely he must hate me by now?
But instead he offers her his hand, once again he shows her that she is not alone. He is there, PARIS is still there cheering her on! More than ever in fact, despite her failing them. Despite loosing EVERYTHING the one thing she never lost, will never loose is her loyal partner, Cat Noir.