The ex shouldn't be part of the equation at all, which is not the same thing as utilizing your current understanding of human behavior. The problem with many relationships is that people keep comparing everyone to their past relationships (either positively or negatively), instead of seeing each partner and relationship for their unique characteristics. That uniqueness makes every new relationship 'unknown territory'.
The other problem with relationships is that because people keep making comparisons, they 'see' their exes in each partner after that, and subsequently treat them the same way they treated the ex, assume intent and motive that may not be there (but was with the ex), and then wonder why their partners are so tired of being misunderstood. In other words, they project when they should be leaving that baggage where it belongs - in the past. There's nothing l resent more than someone going, "You know, my ex used to say/do that all the time." Well, thank god l'm not your fucking ex, eh?
That doesn't mean we shouldn't be aware of our negative relationship patterns and put a stop to anything that is damaging to our psyche/self, but in regards to negative behaviors, new and unknown territory is better than anything that resembles abuse or neglect of any kind.