After a breakup, we tend to look at our exes as perfect. We look past the issues that caused the breakup and remember only the good parts of a relationship. Even worse, we don't see our own contributions to how the relationship failed. This rose-colored glasses viewpoint can have a detrimental effect on future relationships for a number of reasons:

• "No one will ever be like him." She compares future romantic interests to the good parts of her past relationship. As a result, she searches for (and finds) all the flaws in the new guy, and talks herself out of discovering someone new.
• No self-reflection or transition past the relationship. By mentally staying in a past relationship, she limits herself for the future. She subconsciously stays stagnant in the hopes that her ex might return and they'll be back together.
• "Nothing like my first love." Even established relationships can be affected by an ex long-past. If she still holds a candle for a first love, instead of investing in her present with a current partner, she could sabotage her relationship. She might struggle with unnecessary arguments, tension and disconnection because she'll keep her current partner at arm's length emotionally.