...as soon as I’m sat there I feel like people must look at the dynamic and think they’re the couple.
So he has humiliated you in public with her. They have intimacy and his allegiance is with her.
@Anna, while you are working hard in therapy, he is doing his best to undermine you. You need to address your relationship in therapy, and tell your therapist everything you have told us. This is an unhealthy relationship and you need to clearly see that, and work on the reasons you are tolerating it. Try to recognize if you are repeating a pattern.
You mentioned attachment issues and PTSD. Has PTSD been diagnosed? It sounds like you have indeed come through some difficult times. Nevertheless, your willingness to seek therapy shows strength. Don’t fall into the trap of thinking of yourself as feeble, which will hamstring you. You can move forward, honoring your dignity and individuality. You’ve already begun by recognizing the disrespect shown you by these two people.
Work in therapy on building stronger boundaries and learning positive coping strategies, as well as productive communication/conflict resolution skills. Continue working on your self-esteem. These tools will be a godsend in helping you build your life. Find your interests and your talents. Volunteer and give to others.
You have attached yourself to a selfish man who doesn’t have your best interests at heart. He has already betrayed your trust and minimized your feelings. If you assert your boundaries regarding his relationship with the OW, he will likely deflect blame to you by accusing you of being insecure, jealous, silly, or paranoid, etc. Don’t buy it.
Your best bet to create your best life is to walk away.