What you rehearse in your mind has a major effect on what you believe and how you feel. Usually in the days of courtship you thought about your partner in almost exclusively positive terms. When you pictured them or thought about them you probably smiled and contemplated wonderful things about that person. If not, that is part of why you no longer feel the same about them. The first defense of your partner begins in your own brain.
Many people function as though the thoughts in their mind are outside their control.
Yet if the thoughts weren't created, refined and maintained by you then someone else or something else is running your brain. Perhaps the thought police or aliens have taken over your mind. But if these forces haven't assumed command of your brain it is time you become conscious of running it. The beauty of the fact you run your own brain is you can change how you think and feel. The hard part is you can't blame or point the finger at anyone else.