Worst possible timing

I am a psych nurse with 9 months total of nursing experience. I work on an ACT (Assertive Community Treatment) Team. There are 10 employees on the team including a pdoc and we offer all psych rehab services and case management under one roof (substance abuse, voc, housing, entitlements, nursing). We work with the most chronically, severely mentally ill out there. We provide services in the community. I love my job, love my coworkers, love my clients but not loving the agency so much anymore. Their policies are vague and reactive rather than proactive, and med safety is atrocious, agency wide. It's too much of a risk to my license and I have wanted to get into inpt psych for awhile sooooo....Today I was offered a 32 hr/wk position at a local free standing psych hospital. A lot of our clients now end up there when they decompensate and our team pdoc sees outputs there 1 day a week and does call for this hospital. It is a great organization and their nurses rarely leave. In fact, inpt psych positions don't open up often around here so when one opens, you have to go for it. I accepted the position.Problem is at my current job, our most senior nurse is retiring Friday. The new FT nurse started 2 weeks ago and while she has psych experience , she doesn't seem to eager to jump in and get out into the community or meet the clients much, which is going to be a problem. They will have to replace me with someone who doesn't know what the hell they are doing either. I hate putting my team in this position, I am giving them almost 30 days notice. I sort of feel guilty and am dreading the.conversation I am going to have to have with my boss tomorrow, who literally has been the best boss I have ever had. Any ideas on what I should say?