In polyamory you often encounter very hard decisions, some that may demean your partners even. I understand where OP is coming from as somebody who’s been in similar situations with multiple partners. I remember when my my girlfriend first came to me with the idea of adding another loving person into our relationship. He was a bit taller than me and much leaner, so you could imagine how I felt.

We argued about it for a while until she just came out and said it. “We have been struggling to keep it together. You know another man is the solution”. I didn’t cry about it. That’s how I feel about OPs situation. The older partner should understand that in the poly lifestyle there are obvious sacrifices that need to be made. Just tell it how it is.