You know you love cats, you must do, you adopted her
We’ve all had little accidents in early years. Maybe she’s still really stressed as a result of her past life. Or maybe she has a medical problem? Just asking. Spray the carpet, most carpet cleaning products will do the job, or white vinegar, make a fuss of her, reassure her, show her the litter tray again, think about whether it’s in a suitable place for her. And carry on loving her. You’re obviously a very nice person. Happy Christmas and New Year to you.
I do empathise, been there, done that, etc., etc.,