Basically, just get it started and see what happens. Even on the days I really don't feel like working out, just doing the bare minimum to get started will usually get me doing the full thing.
And if I'm really not feeling it a few exercises in, I tell myself, alright, just go really hard on this next one and you can end early. Usually, I don't even end up quitting. Just take it in little steps.
[0]: https://www.goodreads.com/book/show/17859574-how-to-fail-at-...
I'm a bit surprised how politically charged some people seem to be about this book though. You can see it in the Goodreads reviews, a lot of people giving it one star because it said something they don't like politically. This is not a political book. I don't remember any political commentary from my reading of it, but apparently people have found a sentence here or there that seems to maybe suggest support for a political view they disagree with, so they give it one star. But, it's only a few people doing this and overall this is one of the highest rated books I've ever seen across several sites.
When you decouple the ideas in the book from the author though, and weigh them on their own merits, they hold up, at least for me, and that's all that matters.
Usually my normal workout would be 30-45 minutes of a guided class (peloton, etc). But in days where I'm just not feeling it, I select a bite sized class (~10 minutes). If after that short class I'm still feeling lethargic, then I call it a day, still being quite satisfied that I completed a "full" class. But usually I find that after "pressing play" my energy levels are restored and I end up adding another 20 minutes class on top.
I feel this adds the benefit of knowing that there's a clear early "exit path" in which there is no act of quitting, since you do something till completion.