I am also trying to loose weight.
How are you tracking calories in Vs calories out?
I use my computer, you can use web based fitday.com or buy their software.
I find that if I don't eat every calories I burnt the day before I run out of energy too (and I usually don't since I am trying to loose weight) but lately that hasen't been a problem 'cause I only get to ride every other day right now.