Am I overthinking my girlfriend's past or are my feelings valid?

My girlfriend (both of us are in our late 20s) and I have been together for 7 months, and I genuinely love her. I see a future with her, and she has always been loving, caring, and honest with me.
From the beginning, she told me about her past. She had two previous relationships and one situationship. She only had sex within her relationships and never had casual sex (one relationship 4 years and another was FWB for 3 months and then 7 months dating with live in) .
Today, she told me one more thing that she hadn't mentioned before. She said that when she was single, she got drunk at a random party and shared a quick peck kiss with a stranger. She told me this herself I didn't find out from someone else.
The problem is that ever since hearing this, I've been having an emotional breakdown. It's not just the kiss. It's everything combined.
I keep thinking:
She had relationships before me.

She has had sexual experiences before me.

She kissed a random stranger once while she was single.

Sometimes she jokingly says things like, "Don't teach the player" or "I'm a retired player." I know she says it jokingly, but after hearing all this, those jokes are affecting me more.

My biggest fear is that I'm just the "good guy" she settled down with after having all her experiences. I can't stop thinking that maybe she had all the excitement before, and now she wants to marry me because I'm stable and a good person rather than because she truly desires me.
The confusing part is that she has never compared me to her exes, never made me feel unwanted, and has always treated me with love and respect. When I'm with her, I genuinely feel the happiest I've ever been. She has never given me any reason to doubt her loyalty.
So I'm struggling to figure out whether:
I'm experiencing retroactive jealousy and creating stories in my head, or these feelings are understandable and point to something I should be concerned about.

I'd really appreciate honest opinions, especially from people who have been in a similar situation.

Thank you in advance!