I feel your pain, op. I had similar happen to me 3 years ago -- friend of 20+ years, so involved (and supported) by myself and my family that other people thought that she was children's auntie etc etc. Got involved with a chap - and then we hardly saw her. (We did actually get to spend ONE evening with them as a couple)
She declined all invitations to meet up (either by herself, or the two of them) always had some excuse reason, whether it was an open invitation (We'd love to see you -- When and where would suit you?) or when I tried to make specific arrangements (even when these offered great flexibility) The best example is 10-days notice of a 5-day period (half-term) She would never commit to anything
I always made it quite clear how much we would love to see her, but to no avail. The stock answer was 'hope to see you as soon as possible', 'when things have calmed down', 'life is manic....manic...just crazy' at the moment. This is somebody who is early retired, has no children. And new partner also not working. Their only responsibilities are their dog and her elderly mother who has health issues. But mother's health no worse than it had been for previous 10 years, when friend was very keen to spend time with us, and to be seen and treated as one of the family.
Long story shortened this went on for several months and then a few days before one of my children's birthdays, she expected to visit with just 5 minutes notice (she had a gift and card to deliver). She tried to do similar 4 weeks later, approaching another family birthday although, to be fair, this was 2 hours notice :-)
This was the last straw, and I let her know how hurt I was. That I had spent months trying SO hard to maintain the friendship, and had got NOTHING. Then she expected to visit at extremely short notice, without any regard to whether we were well, tired or had anything else planned. Also, whilst it was kind of her to remember birthdays, a friendship consisting only of exchanging cards/gifts (with maybe 30 mins chat) is not meaningful at all.
She did not take this well(!), and the comments received a few days later were along the lines of 'how she had been abandoned by her friends', 'her friends were jealous of her boyfriend', 'resented her boyfriend because she was not spending as much time with them as she used to. I could hrdly believe it!