How do I overcome being obsessed with my partner’s past?

Dear readers,
To put this shortly, I have had a girlfriend for the past 2 months and after she opened up to me about her previous sexual life, such as having cheated on her ex, cuz she wasn’t “receiving enough attention” and that she has had sex with 7 people before me, I am constantly being haunted of these thoughts. I start receiving flashbacks of events I haven’t seen myself, such as the whole cheating incident. We are both 19, with me being the more shy and introverted person, having a single ex-partner. She has had multiple ex-partners and “fuck buddies”, and although I acknowledge that this is completely normal, I am constantly being disturbed by inappropriate thoughts about what she has done (cheating and having fuck buddies while being 16-17), which leads to a lot of arguing and unhealthy fights (not physical, ofc). I am writing this with hopes of receiving answers as to how I can fully understand and comprehend the fact that not everybody is as conservative as me and mistakes happen. I am also positive that this occurs due to me fearing that things, such as cheating may have a negative impact on our relationship and the fact that she has had a lot of partners may be a sign of her wanting diversity, thus leaving me a few months from now. Lastly, I would like to say that I know the problem is mine and this has to do with my mentality, thus I am desperate to make a change, which I am hoping you guys suggest.
How do I overcome being obsessed with my partner’s past?
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