Welcome to the forum! There are lots of supportive people here!
I have a few questions which might help me and others understand the dynamics better.
I don't know your age, which isn't really that important, but how many relationships have you been in? How many relationship endings have you experienced?
I'm just asking because I am in my 50's and have been through so many relationship beginnings and endings that I have come to accept them and therefore they are not as traumatic for me as they were in the beginning.
You could say I have learned how to be willing to "let go" and it's meant to be- it will come back.
Do the two of you live together? If so- how long have you been living together?
Did you know from the beginning that she is poly?
Was this something you agreed to when you got together with her?
The key to life is in being fully engaged and peacefully detached simultaneously and authentically in each moment.