I have been talking with a girl over chat for sometime to know each other and we now have decided to involve families soon. She is modest in terms of dressing and religion and does hijab too.

As we have talked, she told me that she was in co-education in school till 12th grade. She did use to talk with her male class mates and had friendship with them too. She also added them on her private social media accounts where she had posted pictures of herself (in hijab, though some were of wedding events so she did have makeup). Even in university or at workplace, she was alright with meeting opposite gender colleagues, having lunch with them or taking a picture with them. She says she has always maintained boundaries though and has never been informal with them. But I do not agree with her as I think that adding them on social media or even meeting them in general is wrong….I don’t know how to get rid of these thoughts or resolve this situation, can you please guide me on that? This anxiousness and overthinking is eating up my peace and I think about it a lot all day, due to which I do not stay happy at all.#

You both need to agree on the boundaries to be set as per Islamic law. This would be from here on. Once you guys agree, you need to trust each other. Doubting each other with that reason would be from Satan.
If she is willing to be adhering to Islamic law, in terms of modesty and chastity, and willing to stay away from friendship with non mahram which is never innocent, at least from males perspective then you should forget about the past and not allow Satan to bother you with it, and trouble your relationship.