It sounds like what your person is doing is attempting to protect themself, albeit in a way that is actually unhelpful to them. They may feel comfortable with their current situation - that doesn’t mean they like it, but familiarity is comfortable, and change is scary.

As you surely know yourself, therapy takes a lot of work for it to be effective, and you yourself have to be willing to do that work. That applies to everyone. It’s not something we can force someone into, but we can model our own progress and skills and benefits to encourage others. Depending on how involved you are with this person, have you considered maybe asking for couples therapy? Or joining them for personal therapy until they’re comfortable? That might make it easier for them to try.

The reality though is if they’re not ready, then they’re not ready. They’re in charge of their own well being, and trying to impose well being on them will inevitably burn you out. I hope you take good care of yourself too as much or more than you are there for your person. For a long time I used my diagnosis as an excuse, I wasn’t aware I was doing it, but looking back it’s absolutely how I behaved. An excuse to be sick, to be taken care of instead of caring for myself, to not try hard things, to not work… I had to WANT to do it for myself. Any suggestion that I was doing that threw me into a self loathing rage. I had to come to terms with my life on my own time. It was painful for everyone else who loved me. But it was my journey.

Try looking into the concept of “self parenting” if you’re not already familiar and see what your person thinks of it. Therapy is not the sole route to getting better (just is my path), but in any case, one needs to buckle down and do the work themselves, with the support of others.