Writing sprints are generally good when you're trying to get the words out. During NaNo, we do something called "chain sprints" where you write for five minutes, pause for a minute, write for five more, pause for a minute, and then write for the last five minutes. With the breaks you get a little processing/refresher time that you wouldn't have if you force yourself to go for fifteen minutes straight. And usually even if you're stuttering through the first round, by the time you get to the last one, the words may be flowing.

I sometimes like to write a conversation between characters. It might not have anything to do with the story or assignment I'm supposed to be writing. Being able to hear their voices, to get their rhythms down, and insights as to what's making them tick are going to be important in the long run. You could even do it as an interview- "What do you think of X?" and then write what your character says.

Give yourself some grace. The words are there. Good luck!