CSA lump a bill on me which may prevent me helping get my son through uni

I’ve had shared care of my boy since the day me and his partner split up when he was 18 month old. He is now 18. The csa sent me a bill for 20 grand of which I was paying 100 pound a month for quite a few years. I finished in the job of which they were taking money from and didn’t here from them for a couple of years. They have since been back in touch and basically I’m pulling my hair out with worry. Im just wondering if you can give me some advice. I have paid thousands in csa payments in the past but I have paid thousands in the upkeep of my boy and I am wanting to support him in his university studies. please help.