hobbyhorse

nouns

a topic to which one constantly reverts
"don't get him started on his hobbyhorse"
hobbyhorse
is a type of (narrower)
an idea that preoccupies the mind and holds the attention
preoccupation
a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride
hobby, hobbyhorse, rocking horse