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Psychoanalysis may offer a bit more lengthy process to arrive at a conclusion. You may want to explore CBT (cognitive behavioral therapy) Obsessive thinking is a sign of depression or even OCD traits. Meanwhile, you can determine when these thoughts started, if something had triggered them, and to address your fear that if you're honest he'd leave you. Try to find out where is this fear of abandonment coming from (past relationship, self fulfilled prophecy, your true desire for a new relationship that you're guilty to admit, etc)
Your anxiety level would also have to be managed whether you're open to trying medication, counseling, hypnosis, natural supplements. It is possible that your obsessive thoughts manifest in order to control the anxiety and may not be fully related to your desire to leave the relationship. Of course, if you feel unfulfilled, want more out of the relationship and he is not able to offer it, it would make sense that you'd want to leave.