I also think in my non-expert experience perhaps you need something to eat. I am at.the exact same point as you. I get to mile 18 to 20 and I'm done, few miles sooner with hills. 18 on roller coaster hills is my best so far and at 16 miles it's been a struggle for the last 2 miles.to home.
26 is my best on flat and I've been doing a lot more 20-24 mile rides and I'm pretty worn out at 18 to 20. I just went to Pittsburgh (live in the middle of nowhere and the city was really interesting.) I was having so much fun checking everything out I was in the saddle almost 3 hours. Figured I'd better head back to the car to get home, went back across the river and with 2 miles to the car, all I wanted was to make it back to the car.
I know I need more water. I only have 1 on the bike and had 4 frozen bottles in the car. I downed 3 of them when I got back. I think I need something to eat too once I hit about an hour and a half.
One thing I notice is, going for a ride really curbs my appetite. I'm not feeling hungry for hours after riding even if I'm hungry leaving work and stopping before home for a ride. I think it would be a forced feeding, but I think I need it.