Thanks so much everyone for being so kind and taking the time to help me think things through. From all of your different perspective, you have put a coherent shape on exactly the issues that have been bubbling around in a tangled way in my own head. Really you are wonderful.
DH is being kind and supportive about the whole thing, and my mum has been great too. I find leaving babies when they are this age really hard, but DH has said how much he is looking forward to minding DD2 and stepping up his role now that the breast-feeding stage is coming to an end, and that is helping to make me more positive. He will never notice that a baby is upset as quickly as me, but he is a fantastic, magical daddy and besotted with all three of them ...
It's the fact that I don't really have the option to say yes or no to the work that has made it tricky and I think, to be honest, it's stirred up a lot of memories of when DS was very unhappy without me when he was 2. DH was unhappy in work at the time and the college I worked at treated DS as an intrusion into the 'vocation' of academic work and his nursery wasn't kind enough. We made big changes then (DH retraining, moving here etc). We emerged much happier. This time I'll be doing work I like, getting time away from DS and DD1 which is no bad thing (3 is crazy as PF says), and making a bit of money to support the financially precarious but lovely life we have here. I will also be able to buy DH a guitar for his 40th out of my own money which is important to me! The rest will go on cavity wall insulation. (And maybe, Dizzy, a wee dress)
The emotional issue of walking out of the room and being 40 miles away for a few hours will have to be tackled, but I'll battle on bravely with that.
PF Before the offer of teaching came up on Wednesday I'd been pottering around the house thinking about the fact that we've all moved beyond the survival stage of parenting and we're starting to make different choices about sleep, food, work, herd immunity, toys, the family dynamic and all the other stuff that we grapple with as parents of older children. Long may this remain a corner of mn where we can mention the things that trouble us and have our differences respected.
DD2 has lost her fight with a large plastic coat hanger so I need to rescue her without having chance to proof-read my ramble. Thanks again.