Predicting the future actions or behavior of an individual, especially in complex situations involving relationships, is challenging and not something that can be definitively answered. People's decisions and actions are influenced by numerous factors, and each person's situation is unique.
That being said, narcissists tend to prioritize their own needs and desires above others, often lacking empathy for the emotional impact of their actions on others. They may continue a relationship with a mistress if it fulfills their needs for attention, admiration, and control. However, it's also possible that they may discard the mistress if they no longer find her useful or if they encounter someone new who becomes their primary source of narcissistic supply.
Narcissistic individuals are known for engaging in cycles of idealization and devaluation with their partners, which means their feelings and behavior can change abruptly and unpredictably. They might shower someone with affection and attention during the idealization phase but become distant or discard them during the devaluation phase.
It's essential to remember that your well-being is crucial, and being entangled with a narcissist can be emotionally damaging. If you find yourself in a situation where you are affected by a narcissistic partner's behavior, consider seeking support from friends, family, or a therapist who can help you navigate through this challenging time and focus on your own healing and growth. Setting healthy boundaries and prioritizing your emotional well-being is essential in such situations.