I know one of the reasons Marinette didn't really feel comfortable with Adrien wanting to go to therapy was because she's afraid it might make him understand stuff regarding his father being Monarch, but it's not the only one and it's again an interesting character trait of hers. She's always trying to fix every problem for everyone, it's in her nature and it's also the biggest benefit of her power as Ladybug, to fix everything after an akuma fight. She has a hard time accepting that there are things she cannot fix or control, and especially when she feels like she's at fault, hence the thousands of plans she makes up for everything albeit unrealistic a lot of the time. It's one of her biggest sources of anxiety. So of course it's hard for her to hear that Adrien especially might want some external help to feel better - she's his girlfriend so she should be able to fix him and it should be her role to, right? Especially because she's Ladybug and is involved in parts of the source of his current issues - not that he's aware of it. And especially because she feels guilty about the whole secret thing, so that's another reason as to why she thinks that she should be the one to fix his problems, the one responsible to fix them. And thinking that she might not be enough to Adrien feels like a failure to her. Alya explained the difference between a friend/girlfriend and a therapist's role really well in that regard.
It's both a quality and a flaw she has that has always been present in the show and that I feel has been developed more and more in the recent seasons / episodes. It's often been shown how her tendency to want to fix everything and everyone can lead to disaster or make a situation worse (just like, at the right time, it's been shown very often to have a hugely positive impact on people) - she just needs to learn what is hers to fix and what isn't, and know when to step in or not, otherwise she'll drown in her anxiety and guilt - it's also tied to her need to have control of every situations. And that's def something that she could work on....in therapy for instance π«£ I really hope we'll get to see her realize that she needs it too later in the season!
