Maybe even after many rounds of the Mirror Exercise, it still appears as though your Twin Flame is with their third party, happy and settled down with them, and this triggers deep insecurity or upset in you.
Remember, this is your perception of their relationship, and does not necessarily have any resemblance to real life.
In this scenario, your Twin Flame is simply showing you where you are together choosing separation consciousness, or contrast.
In spiritual Truth, you are One and not separate in any way, and your inner hearts are most fulfilled when you are together on all dimensions; spiritually and physically.
Thankfully with the Mirror Exercise, the third party situation never has to bother you. Instead, you can see it for what it truly is – a perfect mirror for you to use to root out every last bit of separation consciousness in your beloved Union.
Third party situations are always temporary because they have no real grounds to exist and build upon. Not when compared to the unstoppable fortress of Love that is Harmonious Twin Flame Union.
All third party situations are simply tools, guiding all parties back toward their Beloved (you to your Twin Flame, the third party to their own Twin Flame).