Why the quick new relationship hurts so much
When someone begins dating soon after leaving you, it can look as if they replaced the whole relationship in a few days. That conclusion makes the breakup feel even more personal. But speed alone tells you very little about the quality of their new connection. You are seeing the outside of a relationship while trying to explain the inside of your own.
A new person can offer novelty and fewer shared conflicts. Early dating often asks less of both people than a relationship with history. That can feel easier without proving that the previous relationship meant nothing. It also does not prove that the new one is destined to fail. The new relationship may last, or it may not. You don't have to predict which one will happen to decide what you need now.
What can affect the length of a rebound
A relationship's length depends on the two people in it, the fit between them, and how they respond when the easy early stage ends. They may discover real compatibility. They may repeat familiar patterns. They may stay together for reasons that have little to do with you. Attachment style can't give you a reliable timeline for any of those outcomes.
If the new relationship ends, that tells you only that it ended. It does not tell you why, whether your ex understands their own part in it, or whether they are ready to build something different with you. If it continues, you may feel hurt and left behind, but its continuation is not a verdict on your value.
The signs that are worth more than a timeline
If your ex contacts you after dating someone else, don't decide what it means from the first message. Did they acknowledge the breakup without blaming you or the new partner? Can they say what they want now? Are they available for an honest conversation, and do they follow through after that conversation? A late-night message or a story view can mean curiosity or loneliness. It is not a plan to rebuild.
Real change becomes visible in ordinary moments. They keep a plan they made. They can hear that you were hurt without disappearing or turning the conversation into a fight. They ask what you need, describe what they can offer, and act consistently enough for you to judge the answer. One intense reunion cannot substitute for that pattern.
What to do while you cannot know
Checking the new couple's posts can make every picture feel like evidence. Social media is especially poor at showing conflict, effort, or compatibility. If each check makes the breakup feel fresh again, you can stop looking at their updates. Do it because the checking hurts, not to make your ex notice your absence.
Ask what was difficult in your relationship before the new person appeared. What did you need that you could not reliably get? What would have to be different for a reunion to be good for you, rather than simply a relief from pain? Those questions give you a decision to make even when your ex's future is unknown.
If the question underneath this one is whether an ex can miss you while dating someone else, read the guide on when an avoidant ex starts missing you. If you are wondering what a return would actually mean, the three-stage return guide makes the distinction between reappearing and rebuilding clearer.
You do not have to predict when a rebound ends to decide what you would need if your ex came back. The new relationship may last or end. Either way, ask whether the two of you could talk plainly, follow through, and live well together.

