don't really have any real advice, but didn't want to leave you hanging, and we just finally did gradual retreat/rapid return successfully with 19mo DS. For months we'd get as far as sitting by the door, but each time we tried to actually LEAVE it was a disaster, or we'd give in to the screaming (sounded like he was being eaten by foxes - he was desperate), or he'd be poorly, or something else, and we'd be back to lying on the floor next to his cot holding his hand while he fell asleep.
About 4 weeks ago we decided to tackle it again, and started by breaking the hand holding routine, and got to a point where he was happy with one of us sitting in the door way. Whenever he'd cry, we'd pop our head back in, and he'd lie down again. At first this took a while, but eventually he was happy just to see us pop round the door again. And we did lots of "mummy/daddy is downstairs if you need anything/I'm just outside" etc
tried a little bit of controlled crying, so we'd put him down, say good night, leave to cry and go back in after 2 mins, then 4 mins, got as far as 8 mins but we weren't getting anywhere and it was too upsetting (and he was getting to the point where he'd try to climb out of the cot by getting his foot over the side so it wasn't safe to leave him) so we'd just leave, and pop back immediately if he cried, wait til he was calm, then leave again. Like I said, some nights we'd have to do it hundreds of times, but very quickly he learnt. Now we put him down, say goodnight, leave the room and he doesn't blink, and just chats to himself happily before falling asleep, or just rolls over and goes straight to sleep. On the rare occasions he cries a bit, we just do a ssshhh from downstairs, and he's happy with that.
We've got our evenings back and I STILL can't believe it is the same baby, I NEVER thought we'd be able to put him down awake and leave him to it - never ever.
I know there is nothing worse than reading about someone who is doing the same as you, but its 'working for them', but here are a few things that worked for us:
DP did it all - we started it just as I'd done my last BF so I wanted to avoid bedtimes for a few weeks so he wouldn't want milk. I think that DP had more succcess than I ever would have. Even now, DS is a little bit clingier at bedtime and will require one or two more sshhhh from me. When DP does it, he's out like a light.
If he was really upset, we comforted him - cuddles over the side, resettling duvet, more milk in his beaker etc. No point being massively strict - find a balance that works. We just made sure we LEFT after each time.
Don't be too worried about a little bit of crying - if she could talk, she'd be a lot more articulate about what she wants/is feeling, instead all she can do is scream, which in some way makes it sounds worse if you see what I mean.
Finally - you CAN do this, she WILL get it.
Good luck.