I've realized that my polyamory is a dealbreaker and I need to leave my 5-year mono relationship

I have been in a monogamous lesbian relationship with my girlfriend for 5 years, when about a year and a half ago, I realized that I am both bi + poly. I did a lot of research + soul searching, and then 8 months ago I told her how I was feeling. It didn't go well, we swept it under the rug and I've tried very hard just to sit on these feelings. I realized now that for me, it's my identity and likely a deal breaker: how do I have this conversation again, knowing it will end in a breakup?

If you know it will end in a breakup, there's not much else to be done, or scripted, or strategized. It's heartbreaking and painful to end a 5 year relationship, but it's understandable, and normal, to have to do so after learning about a dealbreaker.

You let her know that you've tried monogamy with her, and you've tried to 'sit on' these feelings, and now you have 8 months worth of information about how it feels and whether it's working for her. You explain that, unfortunately, you've realized this is a dealbreaker, and so you need to break the deal, and leave the relationship. This conversation will suck - breakups always do. But it's important and necessary.

Try not to frame it as "you wouldn't let me date polyamorously, so I have to leave - this is your fault." It's no one's fault, it's just how the universe has shaken out. You two were good together for five years, and nothing can change or erase that, but you're not good to continue dating into the future. Give her the space she needs to grieve, and rage. Best of luck. 

seeking advice on: 1) if you’ve cheated on a partner(s) 2) if you were cheated on by a partner(s) 3) how did you handle the conflict/situation? 4) in your opinion, is it possible to move beyond this broken trust to a happy & healthy place?

No, I have never cheated on a partner or been cheated on. I think the main issue here is to ask yourself what your key dealbreakers are. We’re taught by society that “cheating” is the cardinal sin in relationships, and most of us take for granted that it’s the mightiest of dealbreakers.

I’ve seen people end long-lasting, loving relationships over their partner getting drunk and making out with someone else at a party. And I’ve seen people stay with partners after they snooped in their email or lied to them about being on vacation. (Personally, this baffles me.)

I certainly have a list of behaviors that I absolutely do not tolerate in a partner. And I have ended relationships after men I was dating violated one of these agreements. I’ve felt let down, betrayed, and disappointed. But it just so happens that none of my Unforgivable Sins in relationships are “don’t sleep with someone else.”

So you need to ask yourself: what are your unforgivable sins? When someone in a monogamous relationship cheats, I think the focus should be you violated our agreement rather than you performed a specific sexual behavior. What’s wrong about cheating is that it involves a breach of trust and deception.

It’s up to each individual whether that’s something you can heal and move on from or whether that specific act is unforgivable. And that takes introspection and the courage to think outside the terms we’ve been handed by our culture.