Hi Boxmonkey!!
I have Mimosas, but they're the
trees, not the "sensitive
plant" that you have. Mine would live through anything!! Here in the South, they are sometimes considered invasive, but I love 'em.
The info I have on your Mimosa says that the
plants need very high light, and constantly moist, but not waterlogged, soil. If yours is suffering you're right, it's probably not getting enough light.
The
seeds germinate quite well, make sure they're completely dry, and store in a cool, dark place until ready to sow. Soaking helps them to germinate more quickly, pour boiling water over them and allow to soak for 2-3 days before sowing.
Nikkal