I am fully supportive of polyamory when it is healthy and consensual. However, I get the impression that you have been backed into a corner on this issue without your consent. You've already stated that you are monogamous and uncomfortable with the whole situation. For her to ask for your support while she grieves this other relationship is entirely unfair. I do not think it would be cruel of you to take a step back from the situation. It's entirely up to you whether you choose to continue the relationship or not, but please do not take the burden of healing her upon yourself. It was reckless at best and cruel at worst for her to put you in this situation, and it is not your responsibility to heal her broken heart. She made the choices she made, and it's important for you to take the steps you need to be okay.