My thought is that pressuring her to get therapy mostly comes out as being invalidating, which fuels the fire of this mental disorder... . driving worse behavior on her part.
Has she not heard 100 times from you that this would help her already? Saying it one more time won't make the lightbulb go on inside her head and change her mind.
Just keep on using the tools you've learned here the best you can, and take good care of yourself.
If you can manage some honest curiosity about why she quit the DBT group--from the perspective of simply wondering what is motivating her... . avoiding any ulterior motives of "correcting her messed up thinking", asking her about it might be validating.
I feel like her sticking with some kind of therapy is a REQUIREMENT for me to continue in this relationship. Yet I also feel that if I try to set a boundary on this, it will feel like an ultimatum to her (and it would be), she will rage, and I might as well just end it.
The boundary enforcement would be ending your r/s because she isn't in therapy. (protects you)
The ultimatum is telling her that you will end the r/s unless she is in therapy. (attempt to control her)
I'd suggest you set your requirement to be better behavior on her part (which therapy could help her achieve). Consider this: She could attend T, but still treat you just as badly, perhaps because she isn't really engaging with it or working on it.
What are your limits for staying in the r/s?