I was recently diagnosed with severe sleep apnea. Should I do durable power of attorney?

Not because of it. It is good to plan ahead in case of sudden illness and incapacitation. Sleep apnea is not an imminently fatal condition, but it does increase the risk of stroke and heart disease.
Sleep ap treatable. Severe sleep apnea is treatable through proper use and calibration of breathing pressurized air through a nasal mask and a ventilator type device for home use. Untreated sleep apnea leads to deprivation of sleep, especially deep sleep and leads to numerous medical conditions such as high blood pressure. Proper treatment should lead to full functioning.