Let us take the following sentence:
You keep saying that I'm too young!
What is the grammatical equivalent in the past continuous? Given that "keep" is a verb, it should be okay to conjugate it, so I would say:
You were keeping saying that I was too young, but recently you changed your mind.
My girlfriend, however, thinks that, as it is an expression, you need to keep it to its initial form:
You were keep saying that I was too young, but recently you changed your mind.
Who is right between the two of us? And if no one is, then what is the right way of using the expression "keep + [verb]-ing" when talking in the past continuous?