So, I've been reading a lot of information online and I think it would be nice to have some type of mental illness
What? Why would you want to have a mental illness?
Multiple personality disorder which is my favorite one sounds really interesting.
There is nothing fun about having a mental illness. You should not want one, it just makes life more difficut.
But it gives you great stories and a good reason to seek treatment.
Would you really want to go through treatment? I'm sure you wouldn't find dealing with an illness as fun as you think.  You might consider becoming a therapist since mental health is so fascinating to you.