My Ds hated his cot and would stand and scream and cry like he wa completely terrified of it or something. To this day I wish I knew about BB then, hopefully I would have come up with a better solution than controlled crying which in the end did'nt really work....in some ways I feel like I failed him, but i guess I did'nt know any better. In desperation I did what was advised, but when that did'nt work for long we decided to put him into a single bed without the legs and guess what? he was happy, we did have to put a barrier gate on his doorway or else he would leave the room but it worked... I reckon try the mattress on the floor, maybe even lay down with her until she gets her confidence back, when she is fully asleep leave the room then to have some me time, also if you don't already have one a night light could make a big difference.
Good luck I remember the feeling well.