At the moment i am in one of this "low energy" phases i have from time to time, its not really depression but just a pure lack of motivation to get things done. I am pretty sure the already shorter days of autumn do their part on this. And 7 months with a very, very active child.
You never really, or at least I didn't when I was in your shoes, recognize how much of yourself you necessarily give in the process of raising a child, a very young one especially. It sounds to me like you're looking into yourself for capacity and finding it's just gone, and this is something I've just come to understand and accept. You'll find that capacity in other ways, and before too long, your child will begin to give it back in turn, but make no mistake: where you are is a high-energy phase sustained over a very long period of time, and you can let yourself off the hook at least a little. :)