The first thing that comes to mind are negative things, but perhaps the reason why your ex found a partner is the fear of loneliness
After a relationship, breakups are usually something that causes emotional damage; but undoubtedly for many the worst thing is to realize that your ex found a partner quickly.
There is always one of the two that suffers more than a separation. Despite stating that there is no longer love, there remains a feeling that you do not assimilate when your ex already has a partner and more if the thunder was recent.
You are looking for an answer because while you are still suffering the loss, he or she seems to have overcome you, did he not feel anything for me? What most are wondering.
The first thing that comes to mind are always negative things, but it is also likely that among the reasons why your ex found a partner quickly has to do with traumas, fears or emotional aspects of him or her, here are some answers
Fear to loneliness
When one has a relationship for a long time, dependency is created and this can happen to men and women, they are so used to having a partner that the fear of being alone leads them to have someone else immediately.
I didn’t love you enough
Realistically it is something that happens a lot, perhaps during the relationship he did not open up so much, he did not give himself if he offered all his love, he had doubts perhaps and that made him not love you completely that is why he quickly replaced you.
I had a secret relationship
Perhaps part of the ending of his courtship had to do with an infidelity which he may have denied you, but it was not like that and with this person who is now his partner it was seen from before and now they make it public.
To forget you
There are those who apply the saying "a nail takes out another nail" which means that sometimes to forget and overcome they have to replace and may not love this couple at all but hope that with him or her they will forget their past relationship.
Just fell in love
Things like love are unpredictable, sometimes when we find ourselves in a somewhat vulnerable point, a person arrives who makes you see life differently, this is perhaps what happened with your ex who fell in love with this person who offers him today what I had already lost with you.
Just as the relationship is two in the separation both must assume responsibilities, and if everything is over it is time to take a break and look ahead and try to fill your mind with positive things, if your ex now already has a partner it is part of the cycle you will probably do it soon in that there are no rules or time.