I hear you--I am so tired, too, especially in the afternoons. I am 16 weeks today and maybe (?) it is getting a bit better, but I find that I can't really stand for very long before I need to be horizontal. I have a four year old, and I will fall asleep while he is playing because I literally can't keep my eyes open. I appreciate the vit. D reminder--I will get better about taking my fish oil and see if that helps. I was like this in my last pregnancy, but that time I didn't have a young child, though I did work full time.
I got through the work day (sometimes holding my head up with my hand and closing my eyes for a while so it "looked" like I was working...so sneaky, I know...sometimes taking a nap in the bathroom...) mostly I think work was okay, but when I got home, I really needed to crash and would rest on the couch until bedtime, which for me then was 8pm. I remember that by week 18 I had my energy back and was talking one hour walks and such. Right now getting to the car feels like an achievement, but I also just keep reminding myself what my body is doing and not to be hard on myself. We'll get through it!!!