You are still single trying to come out of those memories while your partner is happily dating someone! The moment you see your ex moving close to someone else, the first thought that comes to you is "In what way that person is better than me?
You tend to lose sleep and may even feel like crying.
A break up pains a lot and what causes more pain is seeing your ex-partner happily posing with a new person on Facebook pictures.
This is why most rebound relationships abruptly end as people are not yet emotionally ready for a new commitment.
Q: My ex is dating someone else and I heard that they're serious. A: People who just broke up usually speed up things a bit in an effort to bring their current relationship into the same footing as their old one.
Simply put, they are trying to maintain the same relationship but with the a different partner.