I used to be exactly like this tbh. I still tend to overthink things, but now I have a partner that doesn’t mind talking it out with me and communicating (which is how it should be) but we ourselves also need to take steps and meet the other person in the middle. Even yesterday I was overthinking and maybe the version of myself about 5 years ago would have lashed out at that person - instead now I tell myself to breathe and not get mad at that person before they’ve had the chance to communicate with me about what I am feeling. I understand this is not just something someone can immediately snap and do - but it does help when you’re super comfortable with your partner.
I also don’t think it’s necessarily helping looking at their behaviour etc, it’s never nice to see someone you like flirting with someone else - and it’s only normal to feel some kind of sadness or maybe anger about it - but it will never get solved unless you both have a sincere conversation and ask where you both stand in this “relationship” - from what I understand you are not exclusive, and because of that, she may just be keeping her options open or still talking to multiple people. Maybe make your feelings known that even if you’re not dating, you’d like for both of you to only talk to each other exclusively and see where it goes (not necessarily a full relationship).
To sum it up the only way your mind will quiet is when you get an answer about it from the person you’re overthinking about. Starting these conversations can be difficult- but i would only want to be with a partner that communicated with me in the first place.
Also having friends you can vent to helps, and also friends that are honest and will tell you when your overthinking isn’t warranted. It helps me when my friends tell me i’m doing too much instead of friends constantly encouraging harmful behaviour just to be “supportive”.
Good luck. As an overthinker I hate to think someone else is going through the same things. Even though i’ve grown it still tends to eat me up sometimes. ❤️