Was the decision to travel correct after the fiancé's sudden refusal, especially since he was not convinced of it previously? And what is your advice regarding the relationship with the current colleague?

There is no harm in rejecting a suitor without giving reasons. It is permissible to accept the second young man if he possesses good character and religious commitment, with two caveats:

1. It is forbidden for a woman to travel without a mahram. 2. It is not permissible for a Muslim woman to establish a relationship with a non-mahram man, as this is a gateway to fitnah and evil. Indeed, conversing with a non-mahram woman is prohibited for a man.