I can sympathize CAM. I was married to someone who was considerably less mature than me. The relationship was a constant struggle to both explain my motives/logic and to counteract their mistakes. After a few years of enduring the constant disagreements, their insecurities, and repairing the unending damages from their poor choices... the marriage ended.
Sadly it wasn't until undergoing the process of divorce and afterwards that they became aware of their greater immaturity and finally learned a few lessons. The divorce was fair, all debts equally divided. I recovered quickly and without hesitation... whereas they are still struggling with the more basic concepts of self-sufficiency.
I do not know how to encourage someone to mature... other than for them to be forced into situations where there is virtually no way to avoid solving their problems and learning. Experience teaches you to look ahead of the current situation and see where it may lead... and make preparations or reparations so that problems may be prevented, solved, or endured. Trying to teach this concept to someone else who has no experience of it is daunting to say the least.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen