Feeling tired and unmotivated is a sign of depression, but it is also a sign of adrenal fatigue/thyroid difficulties. I thought I was depressed for years and just recently discovered the fatigue has a physical cause, and with treatment for that cause, oddly enough, I don't feel depressed anymore. Maybe you could check out the adrenal fatigue in Health and Healing and see if any of that applies to you (e.g. feeling tired alot, prone to holding onto colds longer than most people, more excess fat around the belly than any other part of the body, etc).
Hopefullly the Lexapro will work. If not, I think a PP mentioned Effexor, and its been my experience that Effexor is the most activating. It can be very hard to wean off of, though.