Dreams about ex-partners often reflect unresolved feelings or thoughts about the past relationship. In your dream, being at your grandmother's house may symbolize a place of comfort or nostalgia, suggesting a connection to your childhood or family values.

Your toddler sister making a mess with your ex's belongings could represent feelings of chaos or disruption in your life, especially related to past relationships. Your instinct to clean up the mess indicates a desire to gain control over your emotions or the situation surrounding your ex.

Your fear upon seeing your ex in the dream suggests that there may be lingering anxiety or unresolved issues regarding the relationship. This reaction could stem from past experiences or feelings of vulnerability.

The incident of waking up and finding that you had lost control (by wetting yourself) may symbolize the overwhelming emotions you are experiencing, possibly related to anxiety or stress in your waking life. It could also represent feelings of helplessness in situations from your past that continue to affect you.

To gain more clarity, consider reflecting on your current feelings about your ex and any unresolved issues that may be impacting your life. It might be beneficial to explore these emotions further, either through journaling or conversing with a trusted friend or therapist. Taking time to address and process these feelings could lead to a sense of closure and peace.