There are two main options that I know of that really work.
1) Positive reinforcement. Give your story to someone else to read, preferably someone who will only say good things about it. The goal right now is to get you to feel good about what you already have and want to write more, ideally for this person to continue to read and praise. (Later you can have a beta reader who will give more distinct and actionable feedback, but that’s for editing.)
2) Negative reinforcement. Write a check for an amount that would hurt (at least enough for a fancy dinner) to a charity that you DO NOT like. Give it to a friend and tell them that if you don’t complete your goal (to write every day until this is done, or to finish by a certain date, etc. Make it a SMART goal) then they need to send that check. I only do this when I need some crazy motivation, but those are the days when writing is the first thing I do, because I CANNOT have that check sent in my name, so help me.