Thank you all for your insights. What in your opinions would prevent someone from getting help if not doing so meant costing them their relationship?
Themselves.
They need to be ready to do the work for their own mental health. How can they work to build and maintain a relationship, when they are unable or unwilling to rebuild and maintain their own heads?
Therapy cannot be done for the benefit of the other person, it's not going to work. If the only reason I was willing to drag myself into a shrink's office, was to only prevent being dumped. It wouldn't be too long before my significant other figured out that nothing is changing about my behaviour. The fact that I was willing to throw hundreds or even thousands of dollars into the garbage, to go sit and make small talk for a couple hours a week. Just to fleece my partner into well... Lip service.
It would just become another lie from me. My intention would not be to hurt my partner, just keep them happy enough not to leave me. I wouldn't mean to hurt them, or to play them for a fool. But that's exactly what I'd be doing. It would also mean that I have so little respect for myself, that the concept of it has become so alien to me I can't even give it to the person I claim to love so much. I would become the very kind of person who I find disgusting. Respectable people don't lie to their partners and manipulate them to get what they want.
If I could ask myself. What would I think if my partner were to begin a new relationship with someone else that acts the same way I do. Would I be okay with that persons behaviour?
If the answer is no, then I can't subject them to an endless barrage of emotional baggage.
If I were to want to become the person worthy of being in that relationship, I need to get help. In which case I'll be able to prove it sometime in the furture. Or I won't.
For your guy? It's put up or shut up time.