I'm not sure where this emotional downturn has come from, but at least today I have recognised it as such; given that it hit yesterday evening, that's not too bad. Given that today's been the most productive day I've had in work since a couple of days before Christmas, it does not feel situational.
I have a number of reasonable approaches, one of which is planning fun things, so expect a planning for the next few months post shortly. I also have a couple of long-shot approaches along the lines of mailing people I've not heard from in years at addresses I am not at all sure still work; relatively little effort and will be very uplifting if they get positive replies.
