Evening littleladybird14,
It's never nice to hear someone's having such an unpleasant time. It's insane, but what you've described is very, very common. I change lives for a living, I do it because I love it and such is my desire to guide, that I have asked some questions below. If you choose to answer them, you may find that things look differently. I hope they help. I understand the answers may be very personal, so feel free to reply directly.
"Since May every month I have been asked to either work extra days or swap my current normal shift pattern to fit in with the business."
What is the most important thing to you in your life right now, and how is that being affected by these changes to, "to fit in with the business."?
"however recently rather than being asked if I can, it feels like I'm being told"
How does this make you feel? What did that do to you?
"I said I could provisionally agree to this but that I would need to review closer to the time."
You're being asked to compromise. In it's nature, that's not a very pleasant thing. What would have happened if you had said, "No"?
"it is actually leaving me out if pocket with additional nursery days or travelling"
What are the benefits of doing what you do?
"So now I've had a snotty email from my manager saying I need to be more flexible"
What does this do to you? What difference would adding this to that sentence make? "So now I've had a snotty email from my manager saying I need to be more flexible - if I want the job"
"I also said I wanted to sign up to classes in the new year with my son on my days off but that if I were to miss these again this would be detrimental to me financially"
This is great. At no point have you not been certain of what you want.
"as well as with spending time with my son, which was the reason I wanted to go part time in the first place"
This is great too! You have your desired lifestyle. How important is it that you live this?
"Nothing was ever agreed in writing regarding these extra days, just my normal shift pattern and working hours. Surely I'm well within my rights to say no aren't i?"
Is this the question you want to be asking?
You're alright.
Love,
Skyler