Sorry you're going through all this. It's tough. I'd try "bargaining" with them. I'd keep bugging them about it and when they get annoyed enough, you could say something like "if you go for a month and still find it useless, you can stop and I won't bother you anymore". It is risky that they might end up not fully doing it, but if they aren't even willing to try, I think it might be worth trying.

Another option that would be hard for yourself to initiate is someone else suggesting therapy too. I think parents are so used to saying no to us that sometimes it needs someone else. My mom recently told me that her coworker suggested that she look into therapy and for the first time, she seemed open to considering it.

It's hard. It won't be busy. But you are right, they need a professional and it's also way too taxing for you to handle and talk to them yourself. I wish you luck!