I feel left out.

Chani • Photographer, nature lover, strong, spirited, kind, plant based, loyal friend. TTC.
Writing this at age 35 I feel as if I'm in highschool again but I guess this is something that can span a lifetime. 
After leaving highschool aged 18 I moved away to another state, leaving my best friends behind. 
I have made some good friends along the way but none like the friends I had in highschool. Those friends and I are so different, living worlds apart now though and live so far away so it's pretty hard to keep the friendships going. 
After escaping a bad living situation in my early 20's I made a really great friend who I did everything with. We even travelled in England, Bangladesh and India together. Since she has had 3 children she has sort of pushed me away with comments about how our lives are so different now. She only does things with other Mums who understand her. (Not that I haven't been supportive and don't get how hard it must be) 
I also have a very close friend who I met about 10 years ago now through work and we are still friends but she is so always so busy so I hardly see her.
So I have a new group of friends who I met through working with one of them and they are all so lovely. I have known them since about 4 years ago and 1 of them I have known for about 5 years now. The thing is though that they all grew up together since they were 13-14 years old so it's hard because 1 of them in particular leaves me out quite often. She will have a girls night and not invite me, or she will post pic with all of them and their partners out at dinner together and my partner and I won't have been invited. 
I feel like I can never truly be part of the gang because I don't have that history with them. 
I really value strong friendships with females and feel like that's something's that's still missing. Does anyone have a similar story? I feel like I'm the only person I know that doesn't have a close friend I can call when I'm feeling down or just drop in to see whenever.