Emotionally drained

Well, I suppose its officiable now (is that a word? spell check says yes). After almost 8 years at my current job (various employers, but the same office, mostly the same people) I turned in my notice yesterday. What I didn’t expect – especially after the giddy excitement of the last few days at getting the offer – was to feel so bad about leaving. I’m more excited than I can put words to about my new job at thinkgeek and the opportunity this is, so why do I feel so drained?