What's The Best Way To Tell Old Employer No?

So this past summer I work at a clinic. I enjoyed the assignment as a contract nurse. The staff was nice to me most of the time, but at times I felt neglected and without direction. Still, I loved being away from the floor. When my contract was over, they wanted me to come back. I tried to explain to the manager over and over again that she needed to call HR and create a per diem position because the agency I was with only does contracts. The manager refuse to understand that concept. She kept telling me I was already per diem with the agency. Whatever... So I waited and came back to the company working in a completely different department 6 months later. I told the people from the clinic that was I was now working with the company, but part time.


I agreed to work one day a week for them. Now it turns out that I can't because the new job requires me to work all 5 days for the next 3 months while I'm on orientation. Also, I changed my mind about working with them at all. I saw one of their nurses and she demanded for me to work for her one whole week while she's on vacation. At first I wasn't even processing what she was saying. Then, I was like is she nuts!? This woman wants me to leave my part time job to cover for her for one week while she's on vacation? Honestly, at that point I said oh forget this. They were willing to let me go in the first place and now that I have a real job (which I prayed about for months) they expect me to throw up everything and come work to work for them. (Why


Anyway, (Sorry about the long info) I honestly don't want to work with them anymore. I just want to focus on my job since it's a new specialty and it's very overwhelming. I want to remain professional about informing the clinic that I can not work for them. What do I say to them? I get the feeling that no matter what I say, they are not going to like it, but honestly I feel that their short staffing issues has nothing to do with me. Should I send an email or call them to say I cannot work for them?


Thanks
"Thank you for thinking of me; however, my responsibilities to my primary job have changed and I am committed to them full time. I will therefore be unavailable to pick up any shifts in the clinic. Please keep me in mind, if my availability changes I'd be delighted to reconsider."