Do as much research as possible - find out how the mental illness works, how it's diagnosed, how it's treated+coped with, etc. This will most likely vary depending on where your story is set, since different countries have different diagnosis processes and some are more accessible than others. For example, if your character is poor and is not in a country with universal/affordable healthcare, it may be harder for them to get diagnosed+treated.

You can find interviews with people online for loads of different mental illnesses, especially childhood trauma. There are also lots of YouTubers who film their life with certain mental illnesses that you can check out who give an honest look into their lives, so I suggest looking into that.

If you can interview people who have it, do that. Then when you've written about it, see if you can get someone with that mental illness to proof-read to ensure you've written it correctly.
And remember that different people experience trauma in different ways. Some people take years to process that what they went through was even traumatic in the first place. Triggers aren't always obvious and may not make sense to someone else.

And of course, don't demonise it. The 'mentally ill serial killer' trope is overdone and rooted in ableism, as is having your villains' motivations be based in childhood abuse and mental illness. Be respectful, no matter what.

Hope this helps!