Mkay.
FWIW, it took me two months to write 500 words. I got a dollar a word as the prize. Whoppee!. Actually the hard part was getting the story down below 500 words. My first draft was about 1200 words. Amazingly the story sounded better and better as I cut out superfluous words and narrowed the focus of the story. When I was done it was EXACTLY 500 words. The winning story was over 500 words, ab out 530 words to be exact. I found out later that the rule for "500 words or less" was "flexible".
Btw, no matter what anyone tells you, everyone struggles with writing. Even the most notable authors do.
When I went to J-school, in the very first class, the instructors started knocking every one of us down immediately. The very first words from our Newswriting I instructor were: "So, you all want to be writers. You're all going to starve." Then she spent the rest of the first class trying to talk us all into another major. She gave every single one of us a "D" or an "F" on the first paper. There were even a couple students in that class for a second time because they'd failed the first time around. All the instructors were tough. People think writing is an easy thing to do. It's not.
So, just write the final chapter from your heart. Then put it away for awhile. You can go back and edit it later. Good luck!