Ykhjuy

I have struggled with mental illness for my entire life. Writing has been an extremely helpful and important mode of self-expression for me, since I was little. But recently, I feel like my writing has reached a point where my stories are all iterations of each other, with similar characters, similar plot lines, and similar endings.

I think this is because the majority of my characters are mentally ill. MC will be full of anger and sadness, with low self-esteem and distorted self-image, just as I feel about myself. When I write storylines about these characters overcoming exaggerated struggles like defeating a supernatural force or surviving an apocalypse, it's analogous to the struggle against my illnesses that I face. And I think that's okay, except I do it for every single story. It's making me bored with my writing and discouraged with myself, because I feel like I can no longer write a character that is unique or layered.

How can I stop forcing my characters into my mold, and create differentiated storylines that don't focus around a mentally ill MC?

the majority of my characters are mentally ill

Is that true, or is it really only true of your main characters, or even of your protagonists? There's nothing wrong with a certain trait or character type being present in many or even all your stories; but if you're worried your stories are too similar, you could change how such characters feature in the stories. In other words, you don't have to sacrifice this topic of interest to you; you can eat your cake and have it. And arguably, it'll lead to even better exploration of the topic through writing.

For example, if I challenged myself to write several stories featuring type-X characters, one could do so with the protagonist, one with their sibling, one with their partner, one with someone they get on with at work, one with someone they don't get on with at work, one with a neighbour, one with a dangerous enemy, one with a more sitcom-style enemy, one with a casual acquaintance, one with someone known to a casual acquaintance...

And that's just options with one type-X character. They differ in the consequences this has for the plot, characterisation, main character's feel and so on, and even the way you write scenes, since you might not have an omniscient narrator. But they're similar in that they each only do X once, which of course needn't be how you limit yourself. Maybe there are two Xs, and their differences pose some interesting questions. (What if one's a hero and the other's a villain?) Or maybe there are several, brought together by circumstances none of them intended, so they thereby learn about each other.

I'm sure at least a few such try-everything ideas won't work for you, or anyone who finds this question later. Maybe one doesn't fit well with the X at hand, or maybe during writing one idea you decide to pivot. That's fantastic! It'll give you new ideas to explore, which is fun however it turns out.

it's analogous to the struggle against my illnesses that I face... I feel like I can no longer write a character that is unique or layered.

Don't worry, you can. Firstly, changing where such illness appears within the story - maybe it's the next character over, or maybe the world itself is radically unlike the one in your last story (what happens to the mentally ill in a utopia, dystopia, alien world, ancient China or Stone Age?), or maybe next time you'll model a character after an aspect of you other than a mental illness (be it as well as, or instead of, addressing such illness).

You're right about the need for unique, layered characters. But you're a unique, layered character. Remember that you-could-do-these list above? Another could be made just as long about aspects of you - your experiences, or anything they've shaped - worth including in a character. Indeed, if you list 10 facts about you, there are 120 ways to choose 3 of them, so there are more options than you think!

One last point: there are lots of other ways every story you write can be something new. Imagine tomorrow your hard drive failed and you lost your longest effort, and decided to write it from scratch. It wouldn't come out the same. Would it be first-person, third-person-limited or third-person-omniscient? Maybe you'll decide to change what you did before. Maybe you'll experiment with countless other variables, whether your next story is brand new or a rewrite of an old one. You can try the epistolary format, non-chronological order, an unsympathetic lead, a mystery, a twist, a setting or genre unlike anything you've done before...

It's okay to have a Blue Period. Picasso's lasted 4 years and was fueled by depression. He produced amazing paintings during this time and it was something he felt compelled to do.

It's making me bored with my writing and discouraged with myself,
because I feel like I can no longer write a character that is unique
or layered.

So now it's time to transition to other things. Having the insight to recognize that is a good thing (and more difficult than you might think).

Take a class. A creative writing class where you're forced to write in different styles, about people and places you normally would never choose, might be just the thing to gain practice writing in new ways.

Co-write a project with someone different from you. Collaboration means compromise. Even if you wanted to make all the characters the same way you have been doing it, you can't. A collaboration can be between two writers, an idea person and a writer, a writer and an artist, or any other combination of talents.

Write some non-fiction for or about people without mental illness. Everybody has emotional challenges and your extensive knowledge of mental illness will help you gain insights into people dealing with everyday life without disability. If you write some stuff geared to a broader audience, it will be good practice in shaping your concepts away from one narrow focus.

Meet people. I give this advice to everyone who asks how they can write a character that's done stuff they've never done, or that fits into a category they'll never be part of. That includes non-disabled people who want to write about mental illness. You can get a lot out of books but it's not enough. You need to encounter a wide variety of people.

In your case, the people you might need to get to know better are in the mainstream. I don't know what your life experiences are or who you hang out with, but my guess is that part of your trouble writing characters not like you is because you don't know (really know) people who aren't like you. This is normal and standard: we all tend to group with people like us.

To be a writer though, we must go out of our comfort zone and hear other people's stories. Not (just) read them. Not (just) view them on a screen. But meet the people and talk with them. Even if your social circle includes amazing diversity in other ways, it may be lacking in the way you need to breakout of your own personal Blue Period.