Apologies for responding to an old thread, but I'm newly joined and excited to participate. :-)
Best advice I've heard for this is - Make a goal that you will write one sentence per day. And write that one sentence no matter what. No matter if you do think it's bad.
The reasoning is - some days you will just sit down and write one sentence, and that's fine. But other days you'll find that sentence turns into a paragraph, or a page, or five pages. But if you never sit down to write that one sentence, you never give the chance for those other days to happen.
That advice came from B. Dave Walters from the podcast "Writing about Dragons and Shit" - which is my second bit of advice to you - find a writing podcast you like and listen to it regularly.
I'm on my third pass through the back catalog of "Writing Excuses", but there's a few I listen to. I get motivation, inspiration, and education from them and I try to listen to at least one episode of one writing related podcast daily. It keeps me thinking about my story and usually makes me daydream about how something they are discussing would work in my story.
And I don’t want to force myself because maybe then it turns out to be just bad, you know?
Also - and I say this as someone who is still a couple tens of thousands of words away from the end of my first draft - you are going to have to revise your draft when you are done no matter whether you think it came out bad or not. So don't worry about that.
Another B. Dave Walters nugget: "The worst thing you write is better than the best thing you don't write." An awesome story that lives only in your head lives only in your head.
Hope this is helpful and good luck! I have a hard time getting started again when life gets in the way sometimes too.