Rabbi Hershel Reichman
The concept of a 'bashert' is not a Halachic concept, so Halachically it is irrelevant, and the Halacha is quite clear that a Cohen cannot marry a convert under any circumstances.
This is the practical bottom line.
The concept of a Bashert is mentioned in the Talmud a number of times. One of the most famous references is the statement that 40 days before a child is conceived, a voice declares in Heaven that this future child will marry 'so-and-so'. It seems, though, that this is more of a theoretical conceptualization rather than anything practical, because the Gemara does not give us any practical clue as to how one can find this Bashert.
It seems though, from a practical survey of things, that nowadays there are many possible people who coud be one's Bashert, and therefore people sort of fall in and out of love, and certainly don't marry every person who could be their Bashert. There are happy marriages and there are unhappy marriages, and with a divorce rate of over 50% in the general American community we live in, it is difficult to find a formula for knowing who is Bashert.
Your story reminds me of the tale of the wife of Potiphar who tried to compromise Yosef. Rashi says that she had a prophetic vision that Yosef woud marry into her faamily. She was convinced that she was his Bashert, and therefore wanted to have relations with him, even in violation of Torah Law for Bnei Noah, which forbid her and him to commit adultery. Yosef adamenty refused her advances, insisting that this would be a crime against G-d, and our Sages state that Yosef indeed felt a huge attraction to her. The fact is that Yosef did marry into Potiphar's family, but he married Potiphar's daughter, not his wife. We learn from the story of Yosef that a great attracion and emotional feelings of love cannot justify violating the Halacha of the Torah and can certainly be misleading and misplaced.
It is very sad when someone has to give up something precious for the Torah, but it is an incredible privilege for someone to sanctify Hashem's name by suc a sacrifice. We can think of Avraham Avinu, who was ready to sacrifice his most beloved son Yitzchak, in order to fulfill Hashem's WIll.
Rabbi Hershel Reichman