Feeling heartbreak from a friendship

I've been friends with person A and B for over three years now. Though friend A and B are closer friends than I am to them and they met each other first. We've been through a lot of things, like their deepest and darkest moments in our life and the happiest moments in our lives. and I know we care about each other as much as I thought we did, until recently I've came to realization I'm just an afterthought friend to them.

It all happened last year when friend A and B would started playing games together, I noticed they would invite our mutual friends, but message me whenever they need to fill out a spot when playing a game, or when they're just chatting in a voice call in ***, friend A would message Friend B in our groupchat, but wouldn't message me. I just thought during that time, maybe they just forgot, so I just joined regardless even if they didn't invite me. 

The conversation and the games felt fine, but I just had that feeling even though we were "good friends" there was this tension I was part of their circle, it was just a feeling, but I wasn't sure, so I just shrugged it off. And this would happen constantly for the past year everytime a call or when they play video games.

Another thing that happened this year was when I was struggling something in my life I thought I could get some comfort or validation, so I would message Friend A and sometimes message in the groupchat with friend A and B.  But when I direct messaged friend A, their response was very short and suddenly they said my struggles stress them out and make them feel tired and I shouldn't seek external validation because I always seek external validiation and it's something I need to fix about myself and I should tell these struggles to my therapist and not to friends. During the moment I understood that, and they were setting boundaries, cause they were also going through something. 

Even in the goupchat with friend A and B, when I voiced my struggles, no one responded. But when it's a struggle between friend A and B, they would comfort and be there for them. But friend A is venting their struggles and it's not considered a tiring and seek external validation?? it's not seen as tiring or stressful?? Aren't they also seeking external validation?? I'm always afraid of voicing my struggles in life, because I was afraid they might think I'm too much, so I never said anything, and this was the one time in an entire year I thought maybe I can be vulnerable and personal. How is it fair friend A and B can comfort each other and it's not seen as tiring and stressful, but when I do it, no one responds to me and when they do it's short sentences and seen as a burden...?

I would always be there for them when they are going through something and comfort them, but I always had a feeling when I voiced my comfort for them, I get shut down, that they didn't want to hear what I said, they only wanted to hear what friend A or B said. I'm not saying they owe me anything, but I just wished I would get reciprocated the same feeling when I went through something instead of being told my struggles make them feel tired and stressed.

Two days ago, the same pattern happened, and it was the last straw. I'm always positive and keep thinking maybe they just forgot to add me in the call, maybe it's not because they don't see me as a friend. I saw friend A in a call, so I joined them, we talked a bit, and friend B and a mutual came in, and shortly after conversation started, friend A wanted to play a game together, and they invited friend B, and two other mutuals to play together. I was shocked, and ??? I was in the call, why didn't they invite me??? They kept talking happily without mentioning me or talking to me...? So I just sat there quietly, but shortly staying there for an entire hour, I just felt extremely drained and sad, and I just suddenly left the voice call and they asked if I'm okay and I said, I'm just tired, but I never told them how I felt.

I know if I told them how I felt it would be seen like it's stressful, tiring, or I'm just self projecting my problems to them, or even playing as a vicitim or blaming them if I told them how I felt about our friendship.  

I can't believe I sent a christmas letter to them saying I love them as my best friend. I was stupid thinking they also viewed me as a best friend. 

I went to sleep last night thinking if I had a good night sleep maybe I can just act happy again with them again and forget what I felt two days ago. But the past two days, everyday I woke up I felt extremely sad and today when I woke up I felt extremely heartbroken, Idk why. This is the first time I ever felt this sad over a friendship...maybe it's because this is the longest I knew a friend for?

I'm probably being a hypocrite saying all this, but I just thought were my friends right, maybe I am the problem? or were they not my friends in the first place? I honestly don't know anymore, it's just a nonstop cycle of this pattern with them, and I can't tell anymore what's considered a friend anymore.