Hi honeybutterbee,
This sounds like one of those annoying moments where what you know in your head (that it's the past, and doesn't say anything about your partner's relationship with you) and what you feel in your heart (sad that your partner's first time won't be with you) don't line up. I hear that you want to be able to
do something about these feelings to make them go away, but honestly I think all you can do is give yourself permission to feel them for a moment and allow them to fade with time. It's okay to feel upset, as long as you're not blaming your partner for their past.
I understand that the intrusive thoughts you're having - imagining the sex your partner had in the past - are upsetting, but this is one of those funny tricks of the mind where pushing them away can actually make them worse. Noticing the thought and then carrying on with what you're doing is actually much more effective.
This worksheet has some good suggestions for how to deal with intrusive thoughts.
You mentioned in your other thread that you're unpicking the impacts purity culture has had on your sexuality - I wonder if those ideas are influencing how you feel about your partner having had sex before?
Also, if one of the things adding to your upset is a feeling that if you and your partner decide to have sex you want it to be special somehow, I can promise you that no matter how many people you've had sex with the first time with someone new can always feel special and important, just like the first time ever having sex can be underwhelming. There isn't a particular first time magic that is then lost forever! Does that help?