ThatGirlInGray asked some really good questions.
I'm going through the exact same thing tonight, so I can relate! My GF is about to have sex, about 5 hours from now, with her first person since meeting me 16 months ago. In fact, as I write this, she's picking out her outfit.
I can completely empathize with your anxiety.... but it will be ok. I promise!
I think we should get a bottle of vodka and share some stories?!
One thing I do know is that things should ideally be as smooth as possible between you two before adding other people to the mix. Have you been exploring poly for a while? Are you mono with her, or do you want to date other people too?
As I said, my GF hasn't slept with anyone besides me yet. But she's been on a ton of dates and has 'done stuff' with quite a few of those people. So I'm quite used to dealing with that, if it helps to share how I deal with you. It's the same way I'm going to deal tonight.
Basically, distract yourself. Keep yourself busy if you can. Try not to play images over and over. My friend reminded me of something very important earlier that really helped with any anxiety masked as anger. She said "Don't hang onto anger. You can't be angry at her for doing what you both signed up for. She's promised you cake. Why are you expecting turkey?" Random... but my friend was totally right.
Often, the idea of it is worse that the eventuality. At least, that's what i've found with her kissing and 'doing stuff' with other people. After about 24 hours, or a night's sleep, everything goes back to normal and the sting goes away.
I personally never feel like a robot about my GF. I never feel nothing. Maybe some people don't get pangs or twinges.... it's not the case for me. But I want her to be free and happy.
My advice is to hang onto the love you have for her. Ride your feelings out. Poke your tongue out at anger, resentment, jealousy.... they are just your demons trying to mess with you. Don't let them.
If there's an underlying issue, like she has 'forced' you into poly, or that you feel you are treated badly, that's different and you will find it harder.
But if she's a good, supportive and loving partner and it's only the poly part... I promise you that you can get through your feelings by keeping busy, reminding yourself of love and calming your feelings, riding them out and sleeping on it.
Ultimately, you could see her sleeping with someone as a good thing. Put it this way... tomorrow, I'll wake up. My GF will have slept with someone who isn't me. I'll be alive. The Apocalypse won't have come. She'll still want to give me kisses and hugs. Everything will feel better. And then I'll know for sure if that I can handle her sleeping with other people. Hopefully it will be the same with you!
The same friend gave me another piece of advice today... (she is Buddhist, so she's like a fountain of wisdom
Her Grandfather always told her "Today is the tomorrow we worried about yesterday - and what do you know? All is well!"
What doesn't kill you definitely makes you stronger
p.s. Hurry up with that vodka....