Navigating Betrayal, Gaslighting, and a Wedding Invite. AITAH If I decline my cousins wedding invite?

I am looking for objective input on a situation that has left me confused and self-doubting. The core issue is a painful betrayal by my former best friend and cousin’s fiancé, amplified by persistent gaslighting from family, and all of them acting like everything is fine.

In 2023, I introduced my new partner (we've been together nearly three years) to my close-knit cousins and my best friend, who I had introduced to the cousin group. Like any new relationship we had teething issues and I told my best friend about these as you would. Things were seemingly fine until our trip to France for my 30th birthday. It was there that things took a sharp turn.

My best friend became intensely hostile and started attaching herself exclusively to my cousins and the fiancé, leaving me consistently excluded. For example, the moment we arrived in France, they all went down to the hot tub without telling me. When I subtlety mentioned it, I was met with the same dismissive line: "You know we love you, just join us." My friend’s excuse for the exclusion was often, "I thought you'd want time with (partner x)," which I never asked for. My friend even started trying to pull attention away from me when I was complimented.

On this holiday from what I know, I’ve never been able to prove it, my best friend and the fiancé began spreading a false narrative: that my partner was controlling me and that I wasn't happy. They held an intervention with me to say so. My best friend even tried to tell off my partner at the airport, calling him a narcissist based on barely knowing him.

When I returned home this narrative was embraced by my family, who intervened, assuming I wasn't wise enough to make my own choices about who I date after spending many years happily alone. No one listened to my actual distress, which was caused by their behavior and the intense emotional control they were exerting over my relationship. The narrative everyone adopted was that “your best friend is just trying to protect you she loves you.”

Things were awkward and weird for the next year as I grew distant from my best friend and she began spending more and more time with my cousins fiancé and inevitably my cousins. They did try invite me a couple times and I did go but ultimately things were not right. They eventually stopped inviting me. Claiming they had made an effort. My cousins finance didn’t reach out to just me once, only via group chats.

I reached out to my cousin and had a heart to heart with him well at least that was my intention. We had been close for many years as we grew up together. It didn’t go as planned as I felt silenced, misunderstood and that I needed to approach the girls to make peace. I told him about my anxiety and he basically told me to get over it and that no one was being malicious.

The climax was finding out they were saying cruel, mean things about me behind my back at a sleepover. This sleepover was 2 months prior. I was told via another friend who had heard from a family friend who was at the sleepover. The family friend had got annoyed about their slander apparently as she thinks I am lovely person and what they were doing was very mean. This family friend has stopped hanging out with them. Despite me knowing this, no one has approached me to apologise and when I confronted my cousin she didn’t deny it.

My former best friend and my cousin’s fiancé are now "like sisters," and I have been entirely dropped. I removed the best friend from my life.

When I brought up any of my valid feelings, the response was always the same: I was "too sensitive", "making a big deal out of things” and “it’s all one big misunderstanding, let’s hug and make up.” This constant denial of my reality and now receiving a wedding invite addressed to both me and my partner is what is making me feel so confused?!

After all of this betrayal, denial, and gaslighting, AITAH if I decline my cousin's wedding invite?