I teach composition at a community college, which means that I spend my working life persuading recalcitrant writers to write.
So I use the same tricks I offer my students.
I set a goal - a word count, a number of hours writing without stopping, anything concrete will do - and then I do it. I don't worry about whether I am writing something good or something bad, I don't worry about where the writing will take me next, I don't wonder what will happen if the writing is not right; I don't do anything but stick to my goal and pat myself on the back when I reach it. Also, I add ice cream.
Choose a small task that you know you can do. Do it. Smile. Wait until later to choose another one.