I'm a little late to the thread (missed lhamo's bat signal somehow). I left my job as an ED of a small non-profit almost two years ago now. I loved the job--had a great staff, a mission I'm passionate about, and a really wonderful and supportive board. A lot of my identity was wrapped up in this organization--it wasn't like any other job I'd ever had, where the professional and the personal were squarely separated.
For me (as you), I wasn't leaving to get away from something or because something was wrong, but rather moving towards something more compelling (in my case, going back to school to get a doctorate in psychology). I, too, gave a lot of notice (six months), because I wanted there to be a smooth transition to the next ED, and I offered to be available to answer questions/mentor (should it be desired) for as long as necessary. My board was supportive--disappointed for the organization, but happy for me. Initially, they talked about my joining the board (even possibly as president someday), and I was keen on this, but I got really excellent advice (here) not to do this, for at least a year. The organization needs to belong to the new ED and it needs to be a clean break. And, if things went wrong (or well!), it needed to be due to the board and new ED, not the old ED.
When the time came, leaving was shockingly easy. I still marvel at how I handed the reins over and walked away. I still care about the organization, am interested, do some volunteer work there on rare occasion, and make donations, but I feel no ownership at all. I am happy for the new ED to take it in new directions, according to her vision and the board's, even if that means some of the things I worked really hard to create fall by the wayside. I'm amazed at how easy this transition has been.
Organizations need new leadership from time to time--I firmly believe that. I had some guilt over leaving at first, but then reframed it as opening new opportunities for the organization to go places I couldn't take them. Time will tell what that looks like, but in the end, I think it was good for both me and the organization.
A lot of the relationships have fallen away, but not all of them. There was a lot of changes among staff and board after the new ED joined (which is typical--doesn't always happen but a lot of new EDs want to make the organization their own). But it's fine, it's good. I am still close to a former staff person, and several board members, and others I met through my work there. And I meet with the new ED every so often for coffee and updates and that is always fun.
And I'm as happy as can be in my new life as full-time student. Giddy, even.