I am sorry you are hurting. Here's some things you could do/think about. YMMV.
1) Could understand GF has to process her stages of grief.
She cannot arrive at instant emotional acceptance after he turns her down. This just
happened. Of course she's depressed. She got rejected. Try not to take any answers like "Maybe, probably" personally. She's digesting stages of grief. The ripple effect of his answer is still... rippling.
2) Could TELL GF that you don't want to break up with her, but if you guys ever arrive there, you like to be broken up with like THIS. So you are not caught blindsided. You seem like you want reassure that ripples over THERE in her other rship isn't going to rock your boat over HERE in the rship between you and her.
3) Could ASK GF to reassure you that she isn't going to dump you and go be with him. That seems to be the root question. Not so much "Are you done with him really?" but "What is going to happen to ME?" So could ask the question you really want answered more clearly.
4) Could ask GF to read this and do page 5 & 6 thing
s, but remember to keep your emotional processing needs in balance with her emotional processing needs. It's hard to break up with someone, do the goodbye sex thing, AND be reassuring another partner who is having a wigginz.
It's also hard to be the partner having a wigginz and want emotional support that your partner is too withered to provide right now. But answer to the higher need first -- who is more broken? I think she is. Could not be trying to wring more
from a bone dry person right now. YKWIM? That doesn't take away from the problems. It just ADDS to them.
5) Do you worry about your GF's commitment to you in polyshipping? Worry about cowgirls/cowboys? Could talk to her about that at some point later down.
6) Get more sleep. Going without isn't going to help here. Tend to your basic self care things so you can be available to support your GF through a hard time. Maybe outsourcing your own UGH elsewhere to this forum, friends, family, etc will help you be more present for your partner first. Then LATER when she's less bedragged emotionally she can give you the support-nurture-reassure you crave from her next.
Emotions are sometimes yummy to feel and sometimes yucky. Internal weather is just weather. You will be ok. Do your best to let it blow on through. Hang in there!