The Lemmy search turned up two mental health communities with many subscribers:

!mentalhealth@hexbear.net - 7.61K subscribers

!mentalhealth@lemmy.world - 7.5K subscribers

Edit: the hexbear one seems dead, despite having more subscribers. The world one seems relatively active.

In 1913, Freud wrote to Jung:

It is a convention among us analysts that none of us need feel ashamed of his own neurosis.

https://www.loc.gov/exhibits/freud/ex/131.html

For Freud, the difference between a "normal" person and a neurotic is not a difference in kind, but a difference in degree. He argued that the same conflicts exist in both; the question is merely whether those conflicts have become pathogenic (disease-causing) and how the person defends against them.

reddit

7cups.com has free lightly trained peer counseling. I audited a bunch of their courses to become a better listener and they are ok. I dunno how helpful it will be but it’s folks who want to listen to you 🤷🏼‍♀️

Mentally ill in what way?

There are some specific communities, depending on what you've got going on. A quick search says there are a few bipolar communities, there are some autism communities, adhd, and I'm sure there are many others, all on Lemmy.

Similarly, several have meme communities that at a minimum give the occasional feeling of "oh hey, it's not just me" and sometimes that's really nice.

I'd say 4chan, but you want it helpful so maybe not.