It isn't generally all that good when you are constantly comparing your old relationships with your new one. Telling yourself that your old relationships are better may indicate that you're not happy with your current one or that you may not be completely over your previous one. However, there is also a downfall in constantly telling yourself that your current relationship is better. If you are truly happy about your current relationship, you wouldn't need to dwell in the past because the present already gives you joy.
Constantly re-living past relationships in your head may also hinder the growth of your current relationship. Telling yourself that your last partner was better will not further your current relationship as you'll keep convincing yourself that the current partner will never be as good. On the other hand, constantly saying that your current partner was better than the previous ones may indicate that you may not be over your past or that there is still some tension left in previous relationships. Unless you begin to enjoy the present with your current partner and experience new levels of intimacy, it will be very hard for your relationship to move forward.
With all that's said, it is almost impossible not to compare old relationships to your current one. We all do it, even the very best of us. But this doesn't mean it's always bad. When we think about comparing relationships, we tend to always compare our partners. But it is actually better to compare yourself throughout your past and current relationship. After every good and bad relationship, we learn a lot about the other person. But most of all, we learn about ourselves a little bit more. We all make mistakes but the ones who learn from these mistakes move forward a little quicker. Instead of forgetting about the love and lessons learned in past relationships, it is better to value the mistakes that were made. The moments shared in your past relationships do not need to disappear from your life completely in order to be with someone else. Rather, accept it as what they were, for that is what makes up the person you are today.