ks
i recently moved to a reduced hours contract but have retained my full time responsibilities.
first off, going back to work is a very big adjustment - it took me three months not to feel completely exhausted. and there was me thinking i'd actually feel better since a day at the office is easier than a day with a small child. so as florenceuk says, give yourself time to adjust.
really obvious one - do you make a prioritised to do list every day? i find ruthless prioritisation is absolutely necessary. also plan your work as far ahead as you can - if you can anticipate any busier periods, would it be possible to bring work forward into the quieter periods?
try not checking your email frequently (very hard!) perhaps twice a day.
this is really going to sound awful, but here goes - see if there are certain requests you don't have to respond to or calls you don't absolutely have to return. if it's really that important - they'll ask/call again, if not, it might just go away altogether.
meetings are what absolutely kill my day. if you find this is happening, check the purpose of the meeting and see if it can be sorted with just a short conversation instead. if people ask for one hour meetings, could you request that it's only half an hour instead?
if you keep a workplace diary - block out a period of time each day to "catch up" with things.
would it be possible to work for say an hour each day other than at your desk? otherwise, people do tend to drop by for chats etc. while chatting to colleagues is obviously one of the benefits of working life(depends on the colleagues of course ), when you are really under pressure, a constant "open house" can really take its toll.
see if you can say "no" politely if you're asked to take on too much.
finally, as florenceuk says, if it's really impossible to manage, you might consider requesting that your responsibilities are harmonised with your working hours.
one last thing, my work is deadline based, with a long hours culture and frequent ad hoc requests, and we are also two team leaders down, so am attempting to cover their work as well as my own. under these circumstances i find i do have to work longer than my contracted hours - just no way round it. if you find yourself in a similar situation, could your dh cover for you occasionally so that you can get out from under? yes - it is a slippery slope!
don't know your line of work ks - if it's not office based then comments above will obviously be a big pile of pants! apologies if this is the case.
best of luck - it will get easier
PS - never say you are leaving "early" if you are leaving at the correct time!