I’ve been studying and teaching courses on deception in close relationships for over two decades. It’s common for people to conceal or mislead their partners about their sexual history. There are many overlapping reasons for this type of behavior.
To begin with, people’s memories change over time. There is a tendency to conceal or highlight or exaggerate events from the past. Additionally, sometimes people blend elements of different events together. While we like to think our memories are mostly accurate, in reality, they can be flawed and change over the course of time. Part of what you may be experiencing is your girlfriend’s shifting recall of what happened in the past. It’s possible that she remembers (or misrecalls) different aspects or facts of events from different times over the course of her life.
In addition to having flawed and changing memories, people also engage in self-deception when it comes to events that are unpleasant to recall. Research shows that many people put a positive (if inaccurate) spin on events, which have made them uncomfortable or embarrassed. Many people believe their version of the truth is real even though it’s not accurate.
People also mislead partners about their sexual history because they are ashamed of what they have done and don’t want to lose, hurt, or disappoint their current partner. People with a concerned style of attachment often conceal information from their partners in order to avoid a partner’s disapproval.
Finally, people with a cool style of attachment like to keep secrets from their partners. They feel that they are entitled to their privacy — they don’t have to answer their partners questions truthfully because they are entitled to keep some information to themselves — it makes them feel like they have an identity outside of their relationship.
The gist of all of this — many people lie about their sexual history.
However, your desire to know the truth, to have a complete understanding of what happened in the past, and for all the details to add up, seems to be placing a lot of stress on both you and your relationship. If this is the case, I strongly recommend talking to a therapist about the stress you’re experiencing. My guess is that there are some trust issues that you should probably be addressed and are coming to the forefront because of your girlfriend’s changing explanations about her sexual history.
Betrayals often bring resolved issues to light and it’s best to cover your bases — focus on your need for certainty (about a gray area of life) and the specific issues that have come up in your relationship. Working through these issues with a counselor is the best way to regain a sense of trust and security.
Hope this helps!