I know addition might get people in to trouble , but 20.000 in to debt in Lineage 2 is new to me also I feel sorry for you I know its not easy to live with debts I had hard few years also (not because L2) ,
Not sure where you are from but there is helplines across the world in each country I presume , also like someone said NCsoft feeds on people like you that's why this game model was introduced in first place,
NCwest wont help you probably they will put next sale with more promising rewards just to push you to buy more,
My suggestion is put a spending limit on your cards like (40$ or 50$ a month on online transactions) to try save yourself from bigger debt and try to find some kind of hobby to distract yourself from having a lot of spare time.
Additional studies helps in this case (like start learning java/python/C++ and etc.)
Helped me when I couldn't sleep due to overthinking my financial situation also got better paid job due to new acquired skills
Hope you will get through this!