Starting over is the fix, but take the knowledge with you:
- In the old chat ask: "Summarize everything we decided for this project: field names, structure, rules, open problems. As a document I can paste into a new chat."
- Check that summary yourself, fix mistakes.
- New chat, paste the summary at the start (or put it in a Project's instructions so every new chat gets it).
Models only "see" a limited window of text. In a 3 week chat the beginning falls off or gets little attention. That's why it forgot the field names.