Can't ride any mobs/minecarts

I was trying to ride a horse when I kept pressing right-click and couldn't. So I thought maybe it wasn't tamed, and used the following command to summon a tamed horse: 

/summon horse ~ ~ ~ facing @s minecraft:on_tame  

But when I tried to ride it, it displayed the "Press SHIFT to dismount" message, but kicked me off as soon as I got on. I don't know if this was something with my controls or something, but it's really weird.