It's fairly normal to have these mood swings towards the end and I went from thinking the thesis coming together to believing alot more work needed to be done. It did turn out to be the latter for me, but it was a doable 'alot more work to be done' (thankfully followed by a straight forward viva and very minor corrections).
Sometimes you feel in the zone, when you are just able to rattle through the work and others where you just hit a major mental block.
Various different approaches work for different people, however, if you timetable the work in such a way you have mini-targets (finish of Lit. Review, Methodolody, Results, Discussion, etc., breaking the bigger bits down further) it may not seem such a big ask after all. Giving structure to your approach may be a big help and many prefer a structure or plan to work to.
I note you're working fulltime. A PhD especially during write-up can be a fulltime commitment in itself. Can you not take a few months off to concentrate on your PhD or do financial commitments preclude this?
Not being allowed an extension because of work commitments is rough but a fact of life. Unfortunately, academia seems to forget that we need money to eat and pay the bills, etc. and life doesn;t stop on their say so. You could play the health card "due to stress" to buy time (but that didn't come from me - well it did, but you know what I mean).
'Keep writing' as per "mhmd"'s comments is a good one and thinking about it, that's what I did.
However, I will add whilst it will be nice to have the PhD, your children come first.
Ian (Mackem_Beefy)