What can I do to manage constant fatigue in pregnancy?

Rest and exercise. Sounds contradictory but regular exercise and appropriate sleep and rest can really combat fatigue.
Be good to yourself. Chronic fatigue is common during pregnancy. If you don't have any health problems that are causing the fatigue, it may be helped by improving diet, starting a regular exercise program, and decreasing external stresses. If fatigue persists in spite of trying these things, talk to your care provider since it could be related to thyroid problems or anemia.