I'm new to this site and was looking for some advise on my situation - I apologise in advance for the long post.
I have two children of my own aged 12 (DS) and 7 (DD) both are and have always been 'good' children, they're polite, have manners, respect their own and each others belongings etc.
DP and I have been together for 18 months DSP was just turned 2 when we met and is now 3 &1/2 - She's very intelligent, can have full conversations and is understanding of everything you say / ask her to do.
When we first met She was lovely, very affectionate and loving, gelled well with my children who both did and still do adore her, She hadn't ever slept in her own bed when we met She wasn't potty trained, couldn't feed herself etc, Still ate very small bits of mushed up 'baby' food for meals, and was generally treated by both DP and his ex as a baby, I've worked really hard with her getting her to sleep in her own bed and have a set bedtime (She goes to bed at the same time as my DD which I do feel is unfair as DD is older but I let that go) and she's done really well with settling in with us as a family but lately (Last 6 months or so) She has been a living nightmare. She breaks DD's toys and when asked why she says 'because I wanted to' She tells lies about DS / DS smacking her, She's told lies about other people doing or breaking things that She's done, She tells her Mum that DP has smacked her etc and says the same to us about her Mum and her partner but once She's confessed to the lie she says 'I just told a joke because I wanted to', She clings to DP and constantly says 'You're only my Daddy aren't you not anybody elses'
If I tell her She can't have something She'll immediately go to DP and ask him and if He is unaware of my conversation with her will say Yes and then she comes back to me smugly telling me Daddy said I can.
She constantly cries 10-15 times a day, and on the odd occasion DP does tell her she can't have or do something she says 'I want to go to Mummys then'.
I understand that She is still very young, but my children have started to notice how she gets away with things 'because she cries and gets her own way' and I feel it's causing a rift between us all and to be honest I feel drained and close to tears every time she's with us - We have her every weekend Saturday morning through to Tuesday morning and every Thursday too - Every time DP has to leave or go out anywhere She has a tantrum and screams the house down and once he's left She's awful with me and says 'I don't like you' or 'I don't want to speak to you' She refuses to do anything I ask her to and just cries and says 'I want Mummy/Daddy' I've tried everything I can to treat her the same and show her the same affection I do my DS & DD but she isn't interested, everything is a massive ordeal with her, eating meals, brushing teeth, getting dressed, She demands to be picked up and carried everywhere or on DP's back / shoulders as soon as He shows any attention to anybody else She's climbing on him and literally holds his face in her hands so He can't even look at anybody else but her.
DP works occasional evenings at the weekend and DSD stays with me and my other children and I hate myself for saying it but I'm starting to resent having her so much when She makes me feel so stressed out. I've started to feel annoyed when my children are with their Dad and we have DSD and I hate that I feel that way.
We're due our first baby together in a few months and all I currently feel is dread. Please help!!