Whatever you do, just keep going.

If you don't have mental capacity to do all tasks today, do one or two. If you want to do that side project you wanted, but lost motivation in the moment, do at least something, like a sign up form. Just keep going. Put some work in, make this day's net impact positive. And it's not all about work! Wanted to play that game you bought on a steam sale but never opened? Play the first level today. Wanted to learn how to make music? Download Ableton or Fruity Loops, watch a tutorial video on YouTube, replicate the steps. Just keep going.

Wandering directionless and letting yourself go is the sure path to misery. Remember — every figment of human behavior has a reason. It is important to identify reasons behind seemingly random behavior patterns and comprehend them in a non-judgmental way. Then, starve what holds you back, and feet what keeps you going.

I have bipolar type I + autism. Using this approach and remembering that everything has a reason helped me debug my low productivity. And no, I don't mean my job, I mean my real goals I want to pursue even if I had a billion in the bank today and never had to work a single day in my life.

Aaand, the reason was?… fear. I discovered I had PTSD all along that manifested when I was misdiagnosed and prescribed strong neuroleptics. In a way, it's a chemical lobotomy, just less invasive and more reversible. My intelligence came back, but it came back together with PTSD.
Now, instead of chasing mythical productivity, I know the reason behind the lack of it — PTSD. It is hard to fight what isn't defined, but it is real to win a fight with a thing with a name and a face.

Just keep going. That's my message to you.