Sunshine firstly very best of luck with your app on Wednesday, take one of your DD with you, long time 4hrs on your own, worrying. You can have a nice chat with your DD whilst waiting & it will form a further bond with her or them, if they both go with you.
I agree it wasn't nice, yes cruel, to be told you will not see your DGC, until you 'spill the beans' just added to your upset. Good job it's now all out in the open and your DD can help you.
You are trying to be strong, all on your own, I'm the same and I too wonder why I have not had a break down, why I cry inwardly all day [5yrs+], but outwardly I've held it back. I would like a doctor or psychologist to answer that one, as it does really puzzle me.
You say you are estranged from your m&d, that is such a shame, it was very good of you to allow your m&d to still have contact with your C, their GC. After 10yrs, can you not reconcile with them? they must be hurting, not having their DD in their lives and you are missing the love of your parents, at a time when they could be helping you and giving you the support & love that you need right now.
Good luck with it all 