I have records of everything we spent
Everything? Do you mean major items like rent and any other purchases like tv or computer? Or do you mean food etc. That sounds a bit much. I mean you wouldn't really expect somebody to pay you back half the grocery bills or money you'd given her to buy clothes, would you??
There are Polish lawyers who specialise in debt recovery but to be honest your chances of getting the money back are slim and of course you'd have to pay the lawyer for their services. She may agree she owes you money but she never signed any agreement with you or had an official loan from you. 15-30,000 zł is a huge amount of money for the average Polish person and if your girlfriend was having trouble just meeting basic living costs when you were living together.......well, need I say more. The best that you'll be able to do, even if it goes to court, is get her to agree a certain amount per month. It could conceivably take her years to pay you back.
Officially she doesn't really owe you anything Simon. She was your girlfriend, you had money, she didn't, you supported her financially. That's kind of a normal thing to do when you're a couple. It's also normal even just with ordinary friendships or with family, for people to say 'I"ll pay you back when I get on my feet' and they don't. That's just life. If you had remained together and maybe got married, would you still expect her to pay you back? I think you should just let it go and get on with your life. My own view on lending people money, is, don't. If someone needs help and you want to help them, give it as a gift and don't expect to be repaid.