I brought up polyamory with my fiance, and he tried to kill himself.

I’m in a committed relationship with my fiancé. Recently I’ve been putting some pieces together and I realized that I am likely polyamorous. I told my fiancé, who has a lot of mental health and self esteem issues, and he tried to kill himself. If I end the relationship I’m scared of what he might do. If I stay, I’ll either remain monogamous and feel like I might be ignoring part of myself. Or I’ll date other people and he’ll continue to hate himself. What should I do?

Any situation where a person makes you feel like their suicide attempts or self harm are in response to your actions, and holds you emotionally hostage by making you responsible for their choices is not a healthy situation. You need professional help, and fast. 

A suicide attempt is serious and not something for you to try and manage on your own. If you haven't already, please seek professional help for yourself, and strongly encourage your fiance to do the same. You can sit with him to call providers, help him look up providers covered by his insurance, etc. - but your immediate goal is to connect him to someone else who is better equipped to support him as a suicidal person. If you're concerned about his safety in the moment, call a suicide hotline or local crisis center, and they can help guide you. Do not take this on by yourself.

Staying in a relationship because you're "scared of what he might do" if you leave is not a good or safe reason to stay. Your goals right now should be to figure out what it would take for this relationship to be safe and healthy for you - if anything. Start a conversation with love but firmness, saying something like "you responding to my honesty about my needs and desires with a suicide attempt should be a wake-up call for us that something major needs to change. We need to figure out what you need to stay safe - you need to start working on these issues right away. I can help you find a therapist if you need me to. And we should probably see a polya-informed couples counselor."

Or, honestly, you can decide that this is enough of a dangerous red flag that you can't do it anymore. You're not obligated to stay with someone just because it would really hurt them if you left. If you feel that remaining monogamous would be unhealthy for you, and your fiance has made it clear that any attempts to even talk about this issue will blow up into crises, it's okay to walk away. Reach out to friends or family who can support him, connect him with a mental healthcare provider, and then take a large step back. You are not singlehandedly responsible for his choices, and you are not his therapist. Don't get stuck believing that you are.