He's a so & so - but now you know this. Most lone parents will go without meals themselves to provide for their kids, sadly many NRP don't. Your spilt is recent, and you now know where you stand. You can only be responsible for your own actions, as a parent - no point driving yourself nuts fretting about another's life choices.
I think you'll just have to accept that circumstances have changed, and alter your perceptions of what constitutes good quality of life for your child accordingly. Drop some of the extra curricular stuff so you can provide the necessities from your reduced income going forward.
Swimming is a potentially lifesaving skill, but ballet and brownies are just "nice to haves". Substitute a play date for the brownies so your daughter still has a bit of a social life, and doesn't feel deprived. Is it cheaper for you to take your child swimming rather than pay for lessons? Get creative and don't let the ex get you down over this - see it as a challenge to find fun, free activities. I shock myself sometimes with what I manage to come up with for DS to enjoy, given my budget limitations and his disability.
Even the cost of uniform can be reduced a bit, if you are prepared to be a bit creative. In the winter kids wear jumpers, so buy plain polo's that aren't logo'd for the time being. Master the art of fitting your own kids shoes so you can utilise the supermarket ranges instead of Clarks/startrite whenever possible. Improve your sewing skills, so that you can snap up second hand items/hand me downs from older friends whenever possible & alter them to fit. Refresh with machine dye if needed. In some areas if you are on benefits you can get up to £40 towards uniform from the council for reception, year 3 and year 7.
Definately double check which items of uniform are compulsory, and which are "nice to have". Do you have to have logo'd PE kit, or will the supermarket range do? Can you get away with NOT buying a dress/shorts for the summer term? George at Asda tends to be cheaper than Next or marksnsparks. Buy a plain lunch box, not a cartoon character one that'll be outgrown cos of peer pressure by 1/2 term.