This is one of those things you offer up to the gods of professionalism. Writing when you don't want to. Well, actually you do it for the good of your career (published or not), but don't let the gods know I went there 'cus we all know what a fickle bunch they can be.
So in sickness, health, gorgeous days, rainy days that lull you into clean sheets and soft down matttress pad, and days when you just can't think of a damn thing to write -- write anyway. Even if it's crap. Even if you know before the first word hits the page that it's all going to be a huge, steaming pile of rhino poo. Write the poo. Trust me on this. Write. The. Poo.
Sure, days come up where ::ahem:: poo happens, we need to take off or just need a nice day outside to enjoy. And it's okay to give yourself permission to do so. As long as you get back on track. Or guess what? The next day it's harder, and the next, and the next . . . And before you know it two weeks have gone by.
It's like changing your eating habits or working out. Writing is a habit you must maintain. It (and the muse) won't always maintain itself. This is self-employment, self-discipline at its toughest. You are your own boss. You set your own hours and breaks and time off. And it's so, so easy to turn a one day break into a three day break into a three month break.
I get so much done when I stick to a schedule. And I have noticed over the years when I do allow myself long breaks of no writing, it's tougher every single time to get back into a routine again. And writing when I feel like it is no longer an option. So, it's back to structure and will power, and daily page count offerings to the gods for me!