What is the meaning of Monsters are real and ghosts are real too They live inside us and sometimes they win?

I do not know exactly what the sentence you want an answer for means. Because it all depends on the context you are using it. Differentiating from religion, beliefs, morality, psychology etc.

But I do believe that Monsters are real(and sometimes they win) has got to do with the physical appearance of wo/men. Translated means that sometimes we enjoy some physical things, like autoflagelation or some sexual desires.

Imagine it like an adrenaline kick that you get by doing certain things that you do while your mind knows it is wrong but that somehow you still enjoy. You are then monsterfiying your body.

In opposite to Monster, there are ghosts, which is more of a spiritual form. The nature of ghosts is seen as the spiritual form of death matter. We can put that in different contexts also, but you may see it as the form of our "mind". In a more romantic context we can think of it as a ghosts that enters our brain and tells us to do say certain things or feel certain ways. This form of taking over is something we cannot feel in the moment it is taking over but only afterwards, when we are thinking about our actions. Think of it like some Alter ego.

Like telling your family that you hate them just to prove one point would be a good example.

It is originally a Stephen King quote.