My 4 1/2 year old daughter seems to have developed separation anxiety ?

Up until 3 months ago my daughter was confident, and happy and was never bothered about me working, even when her father left over 2 years ago it didn't phase her, i met my new partner 12 months ago and my daughter was fine with him, he does so much fun things with her compared to her own dad, and him and his 11 yr old son came to live with us, 6 minths ago, again we disscused it and she was fine with it, but for the last 3 months my daughter has become increasingly insecure, but only during school holidays when I have to work, she spends the time with my partner( who can not work at the moment due to a critical illness) and his son, she will literally cry the whole day saying I just want my mum, and when I come home and ask what's wrong all I get out of her is I don't know I just want you mummy, I don't want you to work, I feel awful seeing her like that and I've tried explaining why I have to work and she promises to be a brave girl next time, but come next time it's the same thing again, I'm at my wits end now, I've tried a reward chart but it is ineffective, I try so hard to have Girly quality time with her but it's the same senario next half term, any advise on this would be a huge help,