my daughter was 19 months when she started going on the potty all by herself too...that lasted about 3 weeks. I didn't change a wet or poppie diaper for 3 weeks, thought I had a phenom on my hands, and with a 3 month old in the midst, I was thrilled to be on my way to no diaps for the older one. Well, after about 3 weeks, she wasn't interested anymore! I kept the potty in the bathroom and one in the car, but nothing. Not sure what the deal was, but with a new sister in the house, I certainly didnt' want to start creating drama over her potty training. About 5 months later, she began again and this time it was for real. She potty trained for day and night at the same time, so we were out of diapers completely practically overnight.
Something I did notice when she was potty training and now as my younger one is starting, whenever they wake up at night crying, they ALWAYS have to go potty. My husband thought I was crazy, and would try and rock them, put them in our bed, rub their back and it never seems to settle them back in for good for the night. Each time I would walk them quietly while they cried to the bathroom, help them on the potty, listen to them pee, wipe them and walk them back, by the time they reached the bed they practically couldn't climb back in because they were asleep already! My older daughter would start to fall asleep as she was peeing sometimes. It was really funny. Once my husband figured it out, he was thrilled to help them to the bathroom. My 26 month old has been going on the potty occasionally during the day, but last night for the first time she woke up saying "i go to the potty now" it did take 3 trips to the potty, quietly, being patient before she went, but right back to sleep afterwards. Good luck.