Nervous about resignation process

I transferred from Tele 9 months ago to emergency almost 10 months ago and the department has been great to me however the place is far away. I applied to local hospitals with no real conviction. However the one hospital I wanted to work for actually gave me an offer and out of sheer awe I accepted. The new place is one of the biggest learning hospitals in the region and I live less than 30 minutes away. Now I'm in my current predicament. I'm so nervous to tell my director/HR about me stepping down (I want to drop to per diem, I still like working there) due to like 7-8 nurses have left in the past two months and they hired only like 3 nurses. Maybe it's weird I have this sense of loyalty to an employer that probably could care less about me, but I still feel like that. The hospital gave me a chance when many others did not, so I feel like I'm stabbing them in the back leaving so soon. Is there a way I could phrase my notice where it's still gracious? And how do I even talk to my director about it?
You are not resigning you are changing your status.Don't burn bridges. You want them to welcome you back just in case this new position does not work out for some reason, or if in a few years you feel you need a change. Write a letter asking to change your status to per diem as of a certain date. Give at least 3 to 4 weeks notice. Explain that you have been offered a position closer to home ( it was a total surprise and you forgot that you had even applied there but dont include that in your letter to change your status) and that the shorter commute will help you to be available to your family, return to school or a reason that applies to your situation. Thank your manager for the opportunity for the valuable experience you have gained working at the faraway hospital.Keep professional and brief. Take a hard copy to HR, they may have a form for you to complete. Keep in mind that your health benefits at the new position may not start until the 1st of the month, ask the cost to cobra and about how you will be paid for any PTO.
I don't want to burn my bridges, the place has been really good to me. I'm going back to work tomorrow so it's gonna be rough. How do I tell my other coworkers that I took the position or do I act like nothing happened?
If someone asks I would say that it was a very difficult decision because I really enjoy working here and working with all of you. I will still work per diem and will be available for one shift per week. That is all that they need to know.You are not responsible for their staffing.