I have times like this and have a different levels of success depending on how bad my day is. My thing is certain tasks seem so overwhelming that i find myself just stuck and unfocused. In general I find it helpful to plan my day in the morning- to kind of schedule the order of how i will do things and start with stuff that's less overwhelming and move onto the more complicated right after lunch break when my mind has some time to refresh.

I also frequently tell myself (quite loudly, but in my head) to focus. It's hard but the longer I do it, the better i am at listening to my inner nagging *****.

On really bad days I will take just a couple minutes out for mindful meditation- where I stay in my cube, even looking at my computer- but I focus on my breath and allow my fears to pope in my head, I acknowledge them and how they make me feel, and then I set them aside. It's weird how much this helps me.

Now my problem is I put so much focus into staying focused at work that there is nothing left when I get home.