I love to write, so I never have to "force" myself to write. I'm happiest when I'm writing, so there's no need. In fact, I often have to force myself to stop writing and do some housework, or whatever.
That said, I do go through cycles with my writing. For awhile I'll write for several hours a day, every day, without fail. We're talking 12 hours a day, here. Then I'll shift and I won't be writing as often. I'll spend more time doing research for my next project, working on my query letter, or my synopsis, or formatting my manuscript. These are all things that need to be done, so I don't criticize myself for "not writing". It's part of the job. Occasionally I'll find myself in an utterly non-writing mood as part of my bipolar swings, and then I'll just work on other (non-writing) projects until I'm back in the writing mood again. It evens out in the end.