I'm a stepmum and a mum.
My dp's ex caused problems from day one. She had far too much to say about absolutely everything we did and stuck her nose into our time with DSD so much that eventually I backed right off spending any time with DSD at all.
Things like saying that I didn't treat DSD the same as my kids, I didn't ask her to do things at home. She told Dp that this made their DD feel left out. So I started asking her to pick up her stuff and put things away and then she told DP I was picking on their DD. DP was around all the time and also asked her to do things, but that was ok. So basically I wasn't allowed to ask my kids to pick something up without asking her to, but similarly I wasn't allowed to ask her to do anything because that was picking on her.
We also had issues with his DD leaving her stuff everywhere. Our house, at home, at school, friends houses. She dropped her mobile phone on the way home once. Without fail, every time, I got the blame for these things going missing. I was accused of stealing phones, chargers, keys, items of clothing. No apologies when it turned up in her mates house or down the side of her bed or was returned by someone who found the thing in the street. Dp did try and keep the peace but his ex was extremely determined that we were both in the wrong. No matter what we tried, it was wrong.
I think the most telling things are two comments she made at separate times to dp. The first was 'you should have got together with someone without children so DD can be the centre of attention all the time' and 'DD is coming to see YOU not that woman and her kids'.
In other words she resented her DD having to share her space with anyone else. She is an only child (although has an older half sister she doesn't live with) and has been surrounded by adults her entire life. She has never had to share anything or her parents, she has never had to live with other children. She has been asked her opinion and given major choices all her life and when I met her (aged 8) she was very much a child who was used to getting her own way and being a 'princess'. (as both her parents referred to her).
I'd like to say things worked out. But his ex moved away with their DD, moved in with her boyfriend, cut contact with DP and died a year ago.
I would like to think I'd be a better ex than she was. I also like to think I'm doing a better job with my own kids but that's another story. I did a lot for DSD, did her hair, took her to school, cooked her food, did her washing, made lunches and took them to school when she forgot to take it from her mum's house. (her mum would ring me and ask me to do it).
I get on great with my oldest stepdaughter and my dp gets on brilliantly with my two dd's. He's a great stepdad.