Yeah, I'm totally with you. I am also good at initiating friendships but super bad at sustaining them in the way that I see others do, i.e. having family-like life-time friendships. It makes me feel very sad that I can't do this.
Very sad indeed - I wake up in the middle of the night grieving about it. And I don't know what's missing in my behaviour. People find me friendly, but beyond the casual enjoyment of each other's company they don't commit.

I have had some conversations about friendship where people said to me "of course friends would back you up about that" and I didn't know for sure...

Also scrutinising my life, I have this odd ability - the creative side of me that doesn't get on with routine - to be attracted to people who admit that they are also bad about sustaining friendship.... They are nice people, not especially neurotic, but don't actually give friendship such a high priority in their lives. Probably because they have siblings.

Will be reading avidly to see whether I can learn anything from your journey .