If you think about it, she's manipulating and controlling the situation. For me, understanding that was an eye opener. It's not that she "needs" you per say, but that she's demanding you and instructing you when you must interact with her. Changing your mindset could help to change the whole situation. When you said it took her 25 mins to let her down, that's not really true. It took you 25 mins to feel comfortable enough to let her be put down. Yes, the cost of doing that is screaming, heartache, and pain for you both, I totally get that (and I am not down playing how horrible it is to hear them so upset), but the quicker you do it, the quicker she figures out she will be ok. Secondly, she has to learn that you can be trusted to come and go, that she needs to understand that she is safe with herself and that you will come back. Enabling her to be dependent on you won't help in the long run.
I don't mean to sound cold or cruel, but honestly, this change of mind set really helped for me. They are so quick to cry, laugh, play, etc, that they recover SO fast when things change. But, of course if they know you will eventually give in, then they will always continue being persistent.
For me, if I had to leave the room and he was upset at that I'd say, "mommy will be right back, I love you", and I'd go. He'd cry and I'd pop my head back in and say, "it's ok dear, I'll be one minute", and go do what I had to (laundry, pee, whatever) and when I came back, I'd go to him and say, "see, I'm back, yay!". And quickly pick up some toys and play with him. I wouldn't cuddle him though, I didn't want him to think that I felt bad for leaving and that somehow I was appologising for the situation. Do you know what I mean? It was more matter of fact, I had to go, I'll be right back, see I'm back, kind of thing. He would cry, get mad, get over it, and start playing. Very soon he wouldn't even bat an eye when I left because I told him I was going and that I'd be back.
That all worked for me, and figured it wouldn't hurt to pass it along, take what you want, if anything, from it.