Well, I think in your shoes I’d also be asking myself whether something I was doing, probably unconsciously, was contributing to what sounds like a pattern of friendships where the other people are far less invested than you. No point calling it a ‘run of bad luck’ if there’s something you can do to alter things in future if you’d like more mutually-sustaining friendships.
One thing that struck me in your post is that you frequently refer to being a ‘third wheel’, to your main university friend being ‘needy’ and that she ‘took over’ and you ‘supported her to my detriment’, then you say you supported another friend who dropped you through IVF, then you say you went wedding dress shopping with another friend without knowing she hadn’t chosen you as bridesmaid, but only the other of the trio.
This sounds as if you’re casting yourself as the supporting act and expecting these acts of service to get you friends. It just doesn’t work like that. If you’re setting yourself up as the shoulder to cry on, you’re rendering yourself semi-invisible, the useful prop who has no life or needs of her own. Exactly the person no one asks to be bridesmaid, godmother, or invites to parties.
I’d be asking myself questions about how I present myself in new relationships, and maybe thinking about centring myself and my needs more when I make new friends in future. Be fussier about who you befriend. Be more about your own life, priorities, needs, preferences. Think ‘Is this friendship nurturing me?’ rather than ‘How can I make myself indispensable to this person?’