[if one had already fornicated with a bf/gf but knows that he/she is not the right one, is it right to break up? or must it be that whoever you lost your virginity to should rightfully become your husband/wife?]

[...] Saying someone is "not the one" is not your place to say, if you have already had sexual relations with the person. You may know that you do not really want to marry X individual, or you do not have "feelings" for this person beyond physical, but there is such a thing as doing what is right even when it hurts.

Remember, "The one" is not a fantastic superhuman floating in the nebula, it is "the one" you marry. Unlike other strange scenarios that can present itself in life, which may require some deep and thorough study of the Bible, your particular situation was actually addressed in the Bible. The solution is provided as well.

It is now your choice whether you will follow that solution.