I am a single mum to a beautiful 9mo DD and my ExDP has started a new relationship after we broke up 4months ago. I'm so upset about this, partly because I was not completely 'over' the relationship but mostly because i am heartbroken at the thought that my DD will be spending time with a new woman in a 'mum' role. I can't bear the thought of it and Im furious that he thinks it's appropriate to introduce her to new people this soon. When i drop her off at his house i spend the whole day with a horrible feeling of anxiety in my belly, it makes me feel physically sick. I feel like my daighter will be happier with them than me coz when she's with me i'm tired, i'm stressed (i also work full time) and sometimes i'm just too exhausted to sit and play, whereas they both get a good nights sleep, have loads of energy and it will be more like a 'family' set up, much more fun for her. I cant tell him he cant have her just coz im feeling insecure but it really does affect me hugely at the thought of it 
I guess all i can do is just learn to live with it..