@NessIllustration I understand the feeling of losing motivation and energy, especially when your work requires it. First I would advise to take a step back and look at why these lulls start to happen, is it work related or something else?
Recently whenever I started to feel like I was heading towards a wall I would just practice speed drawing. Recently I have been using line-of-action.com to do 1 minute animal drawings, which puts me into autopilot mode and gets me loosened up. It's especially useful in the mornings when you don't feel like doing anything.
Another piece of advice is to research other artists work, read their interviews and get inspired. I don't know if it will work for you, but when I find an artist I like it really gives me a boost to get to that level and make me want to get back into drawing again.
It might be a case of feeling trapped in your workspace, for which I would suggest to go do your work at a starbucks or a library or somewhere surrounded by others. The freelancer life can take it's toll working mostly by yourself, but your never alone when you have a forum like ours