@Thifa
I've actually read quite a few stories that deal with that exact issue. Depending on the nation, it takes a few years to complete the princess training, AND SOME ADDITIONAL YEARS FOR THE QUEEN PART. One story the 'villainess' clearly outlines that yeah, she held no affections for the prince, and that she would gladly break the engagement if he really means it but it would take several years, and numerous obstacles before his baron/commoner lover can be queen. First she has to be adopted into a higher house (a count being the bare bones minimum in most stories), then she needs to undertake the princess education, usually the primary reason that the prince doesn't like their fiancés in the first place because they include behavioral training (meaning they will have to act like a typical noble woman), she would also need to be educated in the countries politics and dynamics with its neighbors. Basically you're cramming nearly 20 years worth of education into whatever time frame the prince wants to get married in.
Personally my favorite story was that of a prince dunking on his fiancé with his buddies saying that she was unlike her younger self becoming a typical noble woman that they all hated. We are then given a rant from the perspective of the poor girl about how all this was his fault since he wanted her as his princess and that how even though she has to behave like this, everything else about her is still the same. Like she still has the same hobbies, interests, etc. Anyways, her love for him cooled to absolute zero and hatched a plan to trap him with her step sister. Plan goes off, and later we're given a scene where she married a childhood sweetheart and not being a princess is free to do her pervious hobbies and how the prince is now miserable because he still really loves MC and is even more regretful now that she's back to her old behavior.