There are a lot of reasons for this. Women tend to deal with emotions better. There was a study that showed young girls get over parents divorcing quicker than young boys.
So when it comes to relationships we project our own understandings of emotional situations onto men. We have it in our heads that a grown ass man should know how to act, so when he does something hurtful we believe it's intentional and thus this starts the process of preparing for the end. Notice all of this is internal dialogue. But really this assumption hurts relationships because resentment builds in women instead of communicating. Although men don't help either by being dismissive of a woman's emotions or abrasive or defensive when she does try to communicate.
Moreover, Women have been told by men countless times that we have an expiration date, that men want as much p*ssy as possible...and so on. So women who are ultimately practical creatures oftentimes go into relationships with eyes open to the possibility of it ending. The love they had for the guy is no less real, just because someone is able to bounce back soon afterwards, it's just that there is bigger cultural pressure for women to do so sooner given common negative cultural tropes such as "old cat lady" and so on.