just get over it!!
it makes no sense to go to court / take legal actions - its just not worth your money and time.
write it off as "premium of apprenticeship" or whatever it is called in english - in german we say "lehrgeld" -, if you lose money due to your own mistakes but got an opportunity to learn something.
idk ... write them a formal letter, announcing you are deleting their accounts / accesses / data in 2 or 3 months if they do not pay until n days in the future and move on!!
in the end: large companies have more money, can afford them better lawyers than you - if they are for example a financial-institution, maybe they even have their own business-unit filled with lawyers, who don't have anything else to do than to fight your claims and they are already on the companies payroll...
just my 0.02€