Do what you love to do and be around people you love being around. Ever been tired at work on Friday and then go to a party and suddenly you have lots of energy? Energy begets Energy...its a catch 22...the more you give the more you get...are you putting out energy into anything? Know your self. What makes you enthusiastic? What made you enthusiastic as a kid? Find it again. Dont sleep more than 8 hours a day. Sleeping 10 or 14 hours a day will make you lethargic. Find a positive group with similar interests as you. Many times just having contact with energetic and enthusiastic people will motivate you to be the same. Try www.meet.up.com to find a group with your interests. You can also take The Landmark Forum http://www.landmarkeducation.com/landmark_forum.jsp
Both links will have something in your area if you are in the united states.
Also make sure your surroundings are clean and de-cluttered. Don't wear dirty clothes, bathe daily....all of these things will increase your energy. I am not implying you do not do these things, however I have seen two friends go into the loony bin and a few other in depession and the one thing they all had in common is wearing unclean clothes, not bathing often and living in clutter. Also I noticed they wore a lot of dark colors...mostly all black all the time. Nothing is wrong with black (especially if you live in New York!) it's quite fashionable, but your wardrobe should have choices/varity...everything you own should not be black.
Also get a check up at the Doc asap. You could have cronic fatigue or another medical problem that does not have obvious symptoms. Your iron could be low. Especially if you've been out of the country in the last 3 months. Many illnesses contracted abroad lie dormant for some time before symptoms arise. Get tested for AIDS/HIV and if your a woman get a pregnancy test!
There are 1,000 reasons why you may not be energetic and enthusiastic through out the day. Hopefully my answer will help you find which one (or two or three) it is.