I don't think the OP is wishing to impose her choice upon her DH. At the moment, they have different perspectives and she is thinking out her perspective before putting it to him. Partly because she's afraid he'll say no. She wasn't expecting to be flamed, so she didn't take care to sound responsible and unselfish. I expect she could have framed exactly the same concepts in a way that would have left her less open to accusations of everything under the sun.
We don't live in a society where it's often possible for a 'getting by' family to simply allow mum to retrain and find a job she likes, especially if the breadwinner (who has a satisfying career, incidentally!) is ordering her to take the first dead-end job she can get her hands on. So with a job matching her skill set probably out of reach for now, she's just wondering if maybe the current situation - which has worked tolerably well and in which she is hardly idle - could be continued for a short time in order for her to pursue a dream she's cherished for 20 years. And she's wondering how to put it to her DP so he'll see what she's getting at and they can talk about it.
It's interesting (whole thread is interesting!) how many posters feel it's the automatic right of the DH to be 'supported' by another salary if he wishes to be - and that, if things are 'tight' financially, this means it is the automatic responsibility of the SAHM to return to work in order to make things less tight. This reaction probably wouldn't have been so strong on a different website. Many mumnetters seem to belong to educated middle class families that are struggling to keep their heads above water in a world of crazy mortgages and myriad hidden costs, proud of their right to work but often forced to do so as they desperately try to stay on the right side of the comfortable/poverty-stricken chasm whilst simultaneously managing to achieve an 'acceptable' lifestyle that is often rather aspirational. This is not automatically a 'right' way to live and the OP's DP isn't within his rights to demand it, necessarily; after all, he's appreciated the job she does within the home up to now. (Five hours extra a day, barring sickness and school holidays is not a huge amount of time to play around with and the OP is right to think that writing can be very time consuming. And not everyone has the capacity to survive or produce good work by getting up at 5am, admirable as it is).
We hark back to the good old days when people were satisfied with less but we don't know what to do with a woman who says she doesn't see the point of suffering to acquire more, given that basic needs are amply met. I disagree with posters saying that the OP's DP has the right to demand that she provide more for the household finances. Some posters have felt sorry for that poor man bearing all the burden of being the breadwinner, but in fact he's just going to work, same as he'd be doing if the OP also worked. The pressure of getting by with the salary he earns is something is a shared pressure and needs to be addressed together. Unless he's feeling personally pressurised by his job and would like to leave it for something less well paid, he's not necessarily to be pitied for having to be 'a bread winner', provided he's doing the job of his choice and his partner is fulfilling a purpose within the family that they've agreed is important and meaningful. Until now, that has been the case. Nothing wrong with being frugal, it's not an infringement of anyone's human rights. The OP is not a waste of space, she has been and is continuing to play a vital role within her family unit. That's what is important within a marriage, not who is earning the money. Provided there is negotiation and agreement, and the OP has never (unlike her husband) hinted that she is planning to skip dialogue. In fact, she seems to have come to mumsnet because she's pretty sure she's going to have to fight for dialogue.
Yes, the couple have to agree on their values but neither partner is free to dictate. In a breadwinner/home-maker set up, both jobs should be equally valued with neither partner able to 'terminate' the role of the other. Now, the OP is going to have, realistically, perhaps five free hours each day, barring sickness and holidays.
The OP is uncomfortable with the automatic assumption that she will return to a menial job that she hates, presumably sandwiched between a day packed from beginning to end with childcare and household chores. Quite frankly, I'm also uncomfortable with a group of educated, progressive woman declaring it's her moral 'duty' to the family to do just that. Have we as women moved from a life in which it's our moral duty to 'drudge' at home to one in which we can choose whether to look after our own children - but must then subject ourselves to a life of drudgery outside the home thereafter?