WM, Breaking an engagement is a halachic matter whether one can or not. i.e. See the Chelkas Yaakov and the Chasam Sofer amongst other sources. The fact of the matter is, it is NOT always permissible, and cannot be done without a shaila.
Regarding your latter question: <i>”you really think the person must remained married to a person that they don’t want to be married to?”<i>
That too is a shaila. You cannot divorce “at will” either. Indeed the answer, halachicly, is many times you must remain married even if the grass is greener on the other side.
for italics you want to place “em” inside the arrows not “i”