Some of this is about the difficulty of inertia, which daily writing and daily yoga typically alleviate. it's much less hard to pick up where we left off yesterday than where we left off five days ago.
This is where re-tooling language can become useful, as can bringing in external motivators.
I am a big fan of getting away from the word "have to" in the language stream that rattles around inside our heads. Sometimes I like to replace it with "get to," which is a good reminder that I like my work and life and am lucky to have them both.
As I approach the summer's midpoint and see that I have made fine but not stupendous progress on my yoga and writing goals, I'm playing around with a new rephrasing to see how it animates my work. Rather than "I have to...(write everyday/finish this chapter/get to yoga)," or even the grateful "I get to," I'm thinking about the language of challenging and daring myself: "Can I....?"
Correspondingly, I'm trying to phrase the tasks in ways that are a little more fun: "Can I write about her use of apian metaphors as a narrative problem for Gilman? Can I get Darwin's thoughts on bees into this chapter?" (<--Okay, these phrasings are pretty specific to me, but you can see that it's more fun than "can I finish this F#$*ing chapter?"); "Can I try three new yoga classes before leaving NY?"
My thought is that challenges are fun, they're about rising to the occasion. The have-tos are about returning to earth. As a visual metaphor, I prefer to think about my daily work as something that propels me upward, rather than an anchor dragging me back to earth. A little shift from "have to" to "can I" helps me maintain this perspective.
For another external motivator, check out Kerry Rockquemore's 14 day challenge: