I"m sorry you hurt right now. I mean this kindly, ok?
Your fiancee has decided to break up with you. You want to ask her to be in polyship so she won't leave you.
I think this could be you moving to "bargaining" stage in the stages of grief process.
I think it could serve you better to Google "stages of grief" so you can monitor your your progress through that process as your feelings catch up to your new reality. This is just one example.
It might not be a straight line through it but jiggles up and down til you are done. It takes time.
I think it could serve you better to NOT be making major lifestyle changes (including entering a polyship) at this time while you are mourning.
I think if you want her to stay, you could say "I want you to stay. Could you be willing to reconsider staying?" simply. It is more clear.
I think you could leave polyshipping out of your request. "I want to polyship so my GF won't leave me" is not a healthy reason for you to be polyshipping. Even if it were, you are not a healthy potential partner right now. You are in a mourning time.
You could do your appropriate self care even if it feels "meh" at this time so you can reach healing in time. Get your sleep, eat nutritiously, take walks to destress. If it helps to write out the story, write it out for yourself. If you need to air out or be taken out -- ask someone to take you to a movie. Don't be shy about asking for extra support / comfort from friends and family. Do whatever appropriate things you like to do best in times of mourning.
If it helps you to be ABLE to let go to hope that you might get together again in future... go with it.
But the main bit is you could decide to be ok letting go
right now. You could decide to deal in future at that point in time.
You could decide, and then you could align your behavior accordingly. In time? Your feelings WILL catch up and match your new reality. You could do behaviors that help you move it forward. Could not do behaviors that help keep you in the stuck.
Hang in there.