Declining a job I've already accepted

Hello...

I was offered my dream specialty yesterday. I got an interview way later than expected and did it. I got the job, but I already accepted the other position. To put it into perspective, I was offered a medsurg job and got offered another position in the PICU. the PICU has been my dream...

Below is the letter I think I'm going to send to the hiring and unit manager. "I’m sorry to say that I will have to decline the position at HOSPITALon the UNIT. I was ecstatic and honored to have been offered a position at HOSPITAL; however, I have been offered a position on my dream specialty unit on another unit at a different hospital. The news came yesterday and feel as though I would regret not taking the opportunity in this specialty. I appreciate all the help, guidance, and flexibility you have shown me through the hiring process. I sincerely apologize and wish you, __, and __all the best in the future."

I want to ensure I'm not burning any bridges and want to emphasize that the fact that this is a dream come true is the only reason I'm saying no.

Please help me.