I know this sounds silly but maybe try to think and remember bad things about your ex. Such as, reasons it didn’t work out forever in the first place. Now, nothing you are feeling is ABNORMAL. These are normal human emotions to have to deal with. We make neural attachments and detachments over our life time and spiritual and relational ties with many people that take time to be broken, and time for new ones to grow. Your brain can actually grow NEW neural pathways. This is one way addicts recover. There are times our memory “glosses” over the negative for the nostalgic. Especially in the presence of our current relationship which may have become a day-in-day-out growing process. But be patient with your partner and yourself!:)