That’s an interesting situation, and honestly, I don’t think getting back with an ex is forbidden—it just depends on the people involved. My cousin actually remarried his ex-wife after three years apart. At first, he thought their issues were too big to fix, but with time, they both grew as individuals.
So they got separated due to some issues and got back united after three years. This only suggests that people when they live together do not realize how important are their partners and the relationships between the two. It is only after the separation that the couple realizes the good things between them and misses those old times.
I also have seen people regretting after being divorced or getting separated. In rare cases, they are able to reunite while a majority of the time, the society does not let them reunite and brings cultural and religious hurdles.