My daughter began to self wean around 16 months, and I too was convinced to make it to 18 months for some reason. I also had lots of people telling me babies never self-wean before 18-20 months. I continued to try and nurse her (although only once a day at that point), and made it to 18 months. However - in retrospect it really wasn't worth it. By the time she stopped it had become a negative thing. At 18 months I would offer her every evening like normal, but let her decide instead, and she always said no (very emphatically too I might add!)
I should have listened to her from the beginning instead of fighting her for 2 months. I know it probably isn't what you want, but as I learned - it definitely isn't all about what we want after all! Good luck!