Dreams often reflect our subconscious thoughts, feelings, and experiences. In your dream, several elements stand out that may hold personal significance.

1. Work Environment: Being in your office suggests a focus on your professional life or responsibilities. It may symbolize your current state of mind regarding your job or career aspirations.
2. Co-worker and Son: The presence of your co-worker and son indicates a blend of personal and professional life, highlighting the connections and possibly the tensions between these aspects.
3. Laptop Swap: Your co-worker trying to swap laptops could represent feelings of distrust or insecurity regarding your work or personal life. It might indicate a fear of losing control over a situation.
4. Tablet and Cloud Cover: The tablet with a cloud-like cover might symbolize your desire for comfort and security in your digital life. It could also represent how technology plays a significant role in your relationships and daily life.
5. Ex-Husband and Mistress: Seeing a photo of your ex-husband and his mistress can signify unresolved feelings about your past relationship. The revelation of emails and their discussions about meeting up could indicate lingering emotions related to betrayal, trust, and closure.
6. Gmail Folder "Her": Discovering the folder titled "Her" suggests that you may still be processing feelings of jealousy or hurt regarding your ex-husband's new relationship. It might also represent your need to uncover truths or confront issues from your past.
7. Final Email Exchange: The last email exchange about freedom and happiness points to themes of moving on and the complexities of relationships after divorce. It could reflect your feelings about your ex-husband's happiness and your own path toward healing.

Overall, this dream may be urging you to confront any unresolved emotions about your past relationship and consider how they impact your present. It might also be a reminder to focus on your own happiness and freedom moving forward. Reflecting on these themes can help you gain clarity and peace regarding your situation.