How do I leave a well-paying job I hate for a lower-paying job I love?

I’ve been in this career for more than 15 years, and I’ve just lost the enthusiasm for this field. It bores me now. But it pays well and I have decent job security. My passions lie in areas that don’t pay very well, but are immensely satisfying. I’m going to be 43 this year. If I have to keep doing this for much longer, I think I’m going to go crazy. But how can I justify this change when I have a husband, a mortgage and only 25 years or so left to save for retirement? (We have no children.) Am I having a midlife crisis? Am I just being selfish?