How Do I Tell My Nurse Manager I've Had A Change Of Heart?

I am a nursing student who will be graduating in May. Last June I was accepted for a summer Nursing Internship in the OR. When the program ended in August, I liked the experience so much I decided to work part time as a Scrub Tech while I finished school. The nurse leader who does scheduling, nurse manager, nurse recruiter, and several others have gone out of their way to accomodate me while I have been in school. While I never promised to work in the OR or for the hospital upon graduation, I know all of this was done for me because they were hoping I would. It is no secret to anyone that I planned on going into Forensic Nursing -- I have a Bachelor's Degree in Criminal Justice -- but initially I planned on getting to this career via the OR. However, after working several per diem shifts in the ED and talking to some Forensic Nurses, I've come to the conclusion that ED nursing is the best way for me to acquire the skills I'll need to go into forensics. The hospital I work at now is very small so I have decided to take a position with a larger hospital in the area. I feel guilty!!! How do I tell the nurse manager that I've decided to leave without him feeling like I don't appreciate all that he has done for me? Just the other day he mentioned all of the time the department had invested in me, which made me feel horrible knowing that I was considering leaving. Now that I've made my final decision I just don't want these people to think that I'm ungrateful for all of their help.