This is one of the realities of any relationship, whether mono or poly. At any point, another person may find themselves moved to step away. It is a testament to the inherent emotional bravery of humans that we persist in chasing after connections in the face of this fundamental uncertainty.
Some folks just aren't poly. They may spend time sharing a lover in a poly arrangement, though that's not an indicator of whether or not they are fundamentally comfortable with it.
My most recent ex apparently never got completely comfortable with it. She was happy to spend time with me while still dreaming of finding somebody who lived closer and was mono. It wasn't apparent until near the end of the relationship, either.
You'll have to trust me that the grief passes and you'll be ready to connect again some time after. Until then, love honestly and pack away the good memories for as long as the relationship works.