Do you have a wee interest away from your work other than the gym/exercise ?? When i slide into your similar dip, i delve into my photographs from ages ago (I'm a wannabe good photographer) and start re-cropping or adjusting the filters, etc, to try and improve the photos with a fresh look. It gets me totally immersed for the time i'm doing it, and i feel i've achieved something when i'm done. I think that's the key here, having that sense and feeling about having done something positive. Like JTS98, i sometimes start cleaning one room in my flat, then one becomes two, then the flat's looking good and comfortable again.
Not prying, but is your work restricted and carried out during set hours ?? Hopefully that will keep your focus and attention for the duration. If it's the case that you can do the work as and when you can, or can be arsed, then i think that may be affected if you have the lethargy hanging about in the background.
Onwards and upwards, mate. We all have wee spells where the world can just f*ck off, but it's what's there on the periphery or outside the four walls that are important to you that can push you to having that sense of achievement that pulls you back up to where you normally are or where you want to be.
Lots of positive and great advice been given by the good people of P & B so far. Try some of the suggestions and i'm sure you'll look back feeling proud of what you've done.