M.R.
This will sound counterintuitive but try to exercise if you can. I know, you're probably laughing at me thinking "Yeah right, lady! I got 2 kids and one is only 11 weeks old!". Even if you can manage to sneak in a 15 minute walking session, try to. You may find that it helps your energy levels.
Also, try avoiding junk food, processed foods and fast foods. They're typically nutritionally void and offer nothing for you but empty calories. Go for fresh fruits, veggies, lean meats and low fat dairy. Make sure you're getting enough iron (red meats, beans, legumes, etc.).
Good luck - I hear ya on the energy thing! My newborn is only 5 weeks old and I am totally looking forward to getting up only once a night. Hang in there!