I hear you Op. I once received a wedding invite from a very dear and long standing friend ( had known her over 28 yrs ) that had RSVP by, 2 months AFTER , the rsvp by date had expired. I clearly was offered the place to make numbers/meal had already been paid for. This was a very very good friend of mine, always came to our house after sch, i went to hers when she was grounded. We hung out every day/weekend. Boyfriend troubles shared together, house shared in early 20's together as part of larger group.
I went to the wedding , as it was by the sea ( with my then 4 yr old DS as my +1) and enjoyed the day as did my DS. He played with the other children that day and i saw old friends. We made an extended sea side weekend of it , me and my DS . I booked a cottage, we had a lovely long weekend by the sea side, just the two of us. The wedding day was just one day in our long seaside weekend but , it did hurt me inside ; that i was an after thought and that is was why me and DC were ultimately there , in that part of the world at that time.
I have seen this again and again over the yrs with friendship groups ( i am old now 50 + ) and while it hurts i have learnt not to cut my nose off to spite my face. By that i mean i attend the functions and nights out, (that usually suit everyone else ) but only if it suits me now and i enjoy myself , have a good time and enjoy the social interaction, then i go home. But, i don't "invest" in friends anymore , as i did previously.
There is a similar dynamic with sch gate mums. ( queen bee like, stingy people,own the place, went here as a child, know everyone , never moved away and can make your life hell, if they choose to, knob cheeses type people ).
Occasionally, work colleagues can be the same. Those who have worked together since Jurassic times and , you're not from round here anyway, ( i moved counties) also display this kind of exclusive biaatchy traits. These groups exist everywhere.
When they all have problems ( the only time they seem to want to proactively be with /single me out for contact now ) , i am unavailable . When they have no emergency childcare, i am unavailable now. When they go 3 full yrs without bothering with me at all, then, their husbands have left for someone else, and...they suddenly need to spend a load of time with me.... i am not available anymore. I grew tired of being used and "fitting" in with others. I am there only for the good times now, just like they were.
The positive thing about always being left out and on the periphery is , that you learn to cope and manage life's challenges alone. You become very independent, strong and self sufficient. I am divorced and raised 2 DC's alone while working full time and had no friend /sch mum/colleague support at all , through my divorce and single life journey. My "friends " were always busy with each other , arranging things that , i could not attend with a minutes notice. Had they included me in the planning, i would have been able to attend , albeit with a little foresight and planning and on rare occasion , could have included my children too.
My DC's are grown up now and i am literally living my best life. Most of my " friends" are now where i was yrs ago- ...never mind eh! I think this sort of early exclusion "friendship " dynamic, in my experience, sets a tone for the future. I honestly do not think these friends will be a group , who will be there for you when the chips are down , moving forward and in life. They are their little own group. I do not feel you could ever rely on them for " "life support" if you ever needed it. These kind of "closed" groups are really just that.
Go and attend the good stuff OP, the stuff that others have arranged, if you are free and, enjoy yourself. Then leave it there. Do not waste anymore time organising things, remembering b'days, making people feel special or checking in on them. They are clearly not appreciating it. Those kind of gestures get forgotten in my own personal experience and often in hind sight, then attributed to others . Then you have to remind everyone it was you , did that.
Go to the socials, they organise amongst themselves, have fun , suit yourself and then leave , like an enigma. Do what adds substance and enjoyment to your life. Make other and many additional friendship
groups and have a full social circle /life that suits you.
When life starts squeezing lemon juice all over your current friendship group ...and, it will, you will be in ph neutral calm waters. Leave them to it and look after yourself. Reinvest all that energy and effort into you, your birthdays and your own special occasions. Fook em!