Oh lord.
Anyone with a mental illness will want this trope to die.
In fact, if you're a decent human being, you'll want this to be shunned too.
You know when a character has a mental illness, or a past trauma that affects them? Yeah? Well, naturally it's a major part of their life. So what happens when they find love? They become instantly cured, right...?
Fuck no.
Let's say that Breamwish has social anxiety. Breamwish has fallen in love with Tigertail, and the two are happy.
But that doesn't mean that Breamwish's SAD (Social Anxiety Disorder) is gone. That doesn't mean that he's neurotypical now.
Being in a loving relationship isn't going to help him whatsoever. He will still be scared to talk to his Clanmates. He will still walk with his eyes glued to the ground because he can't make eye contact with someone. He will still take his fresh kill into somewhere in his territory where he is alone because he's terrified of people watching him eat. He will still be the quiet one at the back of border patrols. He'll still spend ages crafting the perfect comment when his Clanmates are having a conversation.
Being in a relationship will not change that. Just because he became comfortable enough around Tigertail to become mates with her, doesn't mean that his terror of his Clanmates will stop.
Echofur had BPD - Borderline Personality Disorder. She is currently in a relationship with Petalfall.
Echofall would not stop over attaching herself to people. Her overwhelming terror of abandonment would not go away, it would continue to manifest itself into behaviours that push people away. She would still have extreme/intense mood swings. "Borderline Personality Disorder as an illness about pain, fear, and struggling to cope with all of that. It's almost like a wounded animal." Being in a relationship would not "fix" Echofur.
It would be a difficult relationship, a roller coaster, but if Petalfall and Echofur try hard enough, they can make it.
Acornface has OCD. And he's fallen in love with Appleclaw. But his OCD doesn't go away just because he met Appleclaw. He still has compulsions and intrusive thoughts. They are still very hard to cope with. He still has to do everything perfectly because if he doesn't, he will be sent to the Dark Forest. He still has to check on his kin constantly because if he doesn't, they'll be killed by badgers. He still has to hoard leaves and shells, because if he gives any of them up, the Clan will be destroyed.
He may love Appleclaw. And Appleclaw loves him. But he still has OCD.
Comfreynose has depression (major). They are in a poly relationship with Sedgetail and Magpiestorm. Comfreynose loves their mates. But Comfreynose still has depression. They are not going to magically recover. They still won't be able to feel positive feelings. They still feel lethargic, and minor (and major) tasks are still very difficult to them, the tasks still drain them. They still feel hopeless. But they love their mates.
While love can impact a mental illness (very slightly) and mental illness will affect love greatly.
But whatever happens, love does not heal mental illnesses.
This is very damaging. I once read a book about a girl with social anxiety. It was going well! Until she met a boy. Then, suddenly, everything was fine.
This made me both furious and sad. Despite social anxiety as being one of the most commonly diagnosed mental illnesses (in the U.S, anyway) there are very little books about it. This was the first one I'd seen! And the author had fucked it up completely.
So please. In your fics, never, ever do the whole "love cures illness" trope. It's deadly.