Now I do not know what to do. Should I take this money and give it in charity on her behalf, or what should I do?.
Relationships between the sexes other than in marriage are among the things that are forbidden in Islamic sharee‘ah. That is because of the evils that result from these haraam relationships, as is obvious to any wise person.
We have discussed previously on this website the ruling on relationships between the sexes and stated that they are haraam, as in the answers to questions no. 47405 and 59907. In the section on relationships between the sexes on this website you will find many answers on this topic.
With regard to what you asked about, the money that you took from this woman, it may be understood that it was a loan or it may be understood that it was a gift. If it was a loan, then it is something that she is entitled to, but so long as she has forfeited her right to it, there is no blame on you for that.
If it was a gift, then it seems that it was a gift in return for your continuing the haraam relationship with her, so it is like payment for haraam work, so you have to give it in charity.
What you should do is give this money in charity, so as to discharge any obligation.
Our advice to you is to cut off all ties with this girl and not contact her again.
We ask Allah, may He be exalted, to accept your repentance.
And Allah knows best.