Retroactive jealousy is simply a manifestation of this fear, and the repeated images and thoughts are your worst fears being played over and over again in your head.
This is why it’s also known as retroactive jealousy ocd.
It’s very important to understand that these repetitive thoughts — going over and over in your mind like a broken record — are, in fact, representations of your worst fear about the present and the future, not the past.
Ironically, you’re not really bothered by any specific ex-lovers in your partner’s past at all; you’re afraid of what they represent.
I’d like to posit that are only two main emotions that fuel this condition of being upset about a girlfriend’s sexual history: Fear and Judgment.