I struggle sometimes as well. My energy levels are very roller coaster, from running and lifting weights in the same day, to not wanting to get off the couch all day. Sometimes I can power through stuff at work, get very passionate about what I'm doing and don't want to get up from the computer. Other times it seems impossible to just get started. I think everybody, whether they admit it or not, goes through this.
What helps me when I get low is to just go through the motions, slowly, and accept that it's okay to not perform 100% all the time. If I am struggling to want to lift weights, I just don't lift heavy, or I do some yoga, or just go for a walk. If I'm struggling with something at work, I just break it down, draw one sketch/feature/surface at a time, or just go do something else till inspiration strikes.
Another thing, is don't be afraid to ask for help. I had a problem with something at work, and it was throwing me really low, and asked my manager for help. I went slow and he helped me out and we got it done. Too be honest, he was really happy with the end result and was glad to help me (something along the lines of it felt good to do something different besides manage for once).
Especially if you're regularly a top performer and you struggle at times, don't be afraid to ask for help. People will most likely be excited to help you, especially if you've given them a lot of help in the past and you don't need help all the time. It lets people know you are human also.