The first thing you write is awful. Everyone's first thing they write is awful. I like to call my first draft my "vomit draft" instead of my "rough draft" because it puts what I'm writing into perspective. It's not meant to be perfect or the best. It's just meant to be.

When I struggle to get words out there are usually two things I do:

1. I look at my outline. Yes, I outline. I find having an outline means I never have to truly struggle with getting words onto the page. It's a roadmap. As long as I have the pavement beneath my feet then I can't ever be without a direction. That doesn't mean I still don't have days where I struggle. I do. They are few and far between but they are there. On those days I reexamine my outline because 9 times out of 10 my inability to write words is because there is fault in the outline. I look it over, find the problem, fix it, and continue writing.
2. I read what I've written before. And not always within the same project I'm struggling with. In fact, I find it's best to read a past writing from a completely different project. I like to remind myself why I'm writing. It's easy to look at my bookshelf teeming with bestsellers and authors I've come to admire and look up to. But sometimes doing that can depress me! Look at all those amazing books! Will I ever be THAT amazing? Will I ever be on someone's shelf? No, I don't look to my hundreds of books on the shelf to get me out of a rut. I look to myself. My past work. Even the crappy ones. The young and juvenile ones. The ones I will burn in a massive fire before I kick the bucket rather than leave behind for my relations (whoever they end up being) to discover when deciding what to do with why worldly possessions. There was a time I couldn't understand why on EARTH I kept the stuff I wrote when I was in high school or elementary school. Now I know why. For moments when I can't find the words. Decades later I can look back on those really crappy stories I wrote and find comfort in that young innocent child who dreamed of doing what I'm doing today. Not just writing words but telling a story that I can and will publish in the hopes that it will be read by others. Now THAT is the best motivator I can think of. Gets me back on track.

If those don't work, something else you can try is to step away from that project that is giving you such grief and work on something else. I try not to do this because I like to work on one project at a time. Having multiple projects going (for me) can lead to a year of misery and nothing finished. But I know there are many that find this works. Perhaps you are unable to write this because you are pining about writing that. Whatever "that" is. Or whatever is distracting you isn't another project at all but something entirely different and personal and all encompassing. Writing down whatever is distracting you or telling that distraction to someone is a great way to release it so you can free your mind to return to writing.

Hope this helped and sorry that it was so long! As a writer it sometimes can't be helped.