I think you did what you could with Russo.
When you ask in your dating process "Are you willing to be in a polyship with me?"
...if the answer is somewhere in the area of "I do not know. I am willing to try though, and find out."
...you could choose to not accept that standard. You have in the past and you are no longer willing to be disappointed in a monoamorous person trying you on for size and coming to the conclusion that they are monoamorous after all and polyship is not their deal.
Because you have chosen as a standard a more enthusiastic "YES! I am willing, I love polyships, and no I will not ask you for monoship. If we break up it will be because of something else not working out. I know my own self and the shape I want my romances to be in and it is polyships for sure!"
Not a guarantee that the relationship will be a runner but then at least you are no longer dealing with THIS part of it assuming person is being honest.
And if a person comes in flying false colors -- that is on them. Not being honest with you or themselves. The only way to not deal in that for sure
is to not date at all. You cannot control other people's honesty. This is the price of admission to ANY dating -- regardless of what relationship configuration a person seeks. If your are no longer willing to take that risk, do not date.
I am sorry you are feeling sad.