I feel your pain. I can ride a bike all day and time seems to fly, but 15 minutes on a trainer seemed to go forever.
I tried watching TV. Meh.
I tried upbeat music. That actually helped ... riding intervals along with the tunes.
Even better was TV with closed captions and the music. But the best I ever was able to do was to ride 90 minutes or so. And it still seemed like a looong time.
I'm lucky enough to live where we can ride all year round. But if I ever get injured again and need to use the trainer again, I think I'll try Zwifting.
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Proud parent of a happy inner child ...