I can totally sympathise (sp?) I have had a similar situation with DD2 - she was going to bed around 8pm, then awake at 1-2am, then 3:30am etc so I'd bring her into our bed and then sleep sitting up feeding her. Its not a good way to get sleep is it?

Anyway, it has gradually been getting better - the night before last she slept through! The only thing Ive been doing differently is getting her to go to sleep in her cot, rather than feeding to sleep then putting her in her cot later

Are you able to get a sleep in here and there? Once a week DH will give me a sleep in and it makes such a difference!

The only other thing I can think of is making sure she's not too hungry overnight (although it does sound like she's comfort sucking) maybe try cluster feeding before her bedtime?

Sorry I can't be more help! Hope you get some sleep soon darl
eta - formula doesn't guarantee she'll sleep better, just thought I'd throw that in there