for me I got this way when I started exercising at first , espcially with depression. Hopefully your body should adapt within a month (I know that seems like a long time) if you keep going reguarly. You could alwyas go and do less than normal the days you are really tired?

For me if my depression was really bad and it was a gym day, i'd turn down the resistance on the eliptical machine I go on (well I haven't gone in 2 months now =( ) and maybe reduce the aerobic time by 5 minutes if necessary. I'd also reduce the weight lifting by about 5 pounds depending on the exercise if I was having trouble. Making the habit, in my opinion, is just as important as how MUCH you do while at the gym.

How's it been going lately?