my girlfriend cheated on me with one of our friends. she told me before she cheated that she had feelings for him, but i told her i was not comfortable with it because i am mono and feel horrible about the idea of her with someone else but she kissed him anyways. she now has decided she is dating both of us without my consent. i really do not want to leave our relationship, we both love each other so much and wanted to spend the rest of our lives together, and i feel like leaving would endanger both mine and her lives. she is not mentally stable but sees a therapist. i don't know what to do, because she says she needs both of us to be happy, but if that happens i’m going to be increasingly depressed. i’m just so lost right now and there doesn’t seem to be any way to solve this.
Your partner cheated on you, is trying to force you into polyamory without your consent, and is holding your emotions hostage by saying that you being happy is a "need" that somehow she deserves to have met. You know that staying in this relationship on these terms will make you "increasingly depressed." Friend, you've got to leave this relationship.
What you want to keep is your ideal, best-case-scenario possibility of this relationship, not the reality of it. What you're holding onto doesn't exist anymore. The sooner you get out, the sooner you can start healing.
It is not okay, and not healthy, to be held hostage to implied, or explicit, threats of suicide. You are not obligated to stay with someone just because their mental health would be impacted by you leaving. It's okay to call your partner's therapist and see if they can help you and her through this; or to ask your partner if you can come to a session with her. You also absolutely need to see someone yourself - please find a therapist asap. Reach out to friends for support. Don't get dragged into a spiral of managing her mental health for her; if she threatens self-harm or suicide, connect her to her therapist, a hotline, or a friend, and then take space.
Check out my mental health resources here, and good luck getting out of this situation. You don't deserve to feel so trapped and unhappy.