Hey hey, we are in a similar but not quite the same situation - we have just moved DD from her cot to the big bed. She is 15mths too... but we don't cosleep and she was generally pretty good at sleeping in her cot... but I wanted to try and help anyway! If I can...

Things I can suggest that worked for us:
- We had the bed set up and encouraged her to play on it before we tried to get her to sleep in it. We didn't want it to be a negative place with bad experiences (ie only sleeping) so we spent time in there reading books, playing with toys, etc.
- She uses a sleeping bag and (at this point) she still uses it in the big bed. So she still has that sleep trigger - "in your bag, time for sleeping".
- We use verbal sleep cues - "Time for sleeping, kiss from mummy, I'm closing the curtains now" - which we try and keep consistent every time she has a sleep (day or night) so she understands it is sleep time
- We moved her sleep toys with her into the big bed

So does your DD have any sleep cues or toys that you could use? My DD always sleeps with a Miffy and a stuffed ferret and as soon as we put her down her hands are reaching out for them, she gets quite worked up if she can't find them. If your DD doesn't currently have a comforter like that, could you introduce one? You could start off by telling her "Mummy and Miffy (or whatever) are having a sleep now, time for baby to sleep too" and then just wean her back to "Sleep time with Miffy" or whatever.

The other thing you could try is telling her what is happening - "Its time for sleeping, lie down and have a sleep now. Mummy will give you a kiss and tuck you in and I will be right here when you wake up."

I'm starting to realise my DD understands more than I give her credit for... so maybe if your DD understands what's going on and feels safe and loved she will be happy to go to sleep in her new room.

Or something else you could try - keep lying down with her, I think you said her mattress is on the floor? You could lie next to her, then when she is dozy but not asleep, you give her a kiss and say "time for sleeping" and leave. Then you gradually leave earlier and earlier... until she is wide awake and you don't even need to lie down with her.

I hope some of these suggestions might work for you... Its a tough time! Good luck!