The first thing I was thinking was, how do you (and possibly they) feel about keeping the relationship open? Polyamory works for a lot of people.
I appreciate the bind you're in. And I wish you all the best. Decisions are tough, just the feeling of having to let go of something that has been the source of pleasure and nourishment in your life even though you get to hold onto something that has been the same.
I have no advice. I can't tell you what to do. I just hope you are able to come to peace with this decision.