Janey,
You are doing some deep work trying to figure this stuff out

I can relate to analyzing and putting the pieces together, it helped give me a bit of control of feeling sad, lonely, scared. The thing is, eventually I had to just accept and feel the sad, lonely, scared.
The great existential questions:
Do I matter? What does all of this mean? etc
So when faced with being alone I haven't got the inner tools, the internalised loving security giving good enough parents and so feel scared and lost?
This is likely true - so what do you do about it?
We "reparent" ourselves. Self talk, letting ourselves feel sad or scared and telling ourselves it will be ok. Treat ourselves like the emotional 3 year old that needs to matter and be validated for our own fears. We put ourselves around trusted friends who accept us for who we are and are "there" for us.
Try to keep in mind, these patterns didn't happen over night - years of stuffing fears and controlling outcomes result in some time for you to learn new tools and strategies for handling emotions. Give yourself the same compassion and empathy and boundaries you would for a small child as you heal.
Good stuff
SB