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Regrettably, based on the information that you have provided here, this is a "losing battle."
If you do not have anything in writing at the time you lent her the money that this was actually a loan and not a gift, you would not prevail in any lawsuit against her to recover the money.
Also, because you gave her this money at the time you were in a relationship with her and never told her that you wanted to be paid back, but now want the money back after the relationship has broken, it would appear that you are only asking for the money back because you no longer have a relationship with her. The presumption would be that you did infact give her this money with no expectation to get it back, but you only want this money now because she has broken things off. The Court would most likely rule against you.
I am sorry that I do not have better news for you.