Should I propose to a girl who previously rejected me for her studies, knowing that I felt comfort and reassurance after performing Istikhara?

If the girl is religious and of good character, there is no harm in repeating the marriage proposal attempt. It is preferable that this be done through intermediaries to avoid embarrassing the family. If her studies are about to finish, the intermediary can clarify this, unless there is another impediment they have not disclosed. In all cases, the matter should be entrusted to Allah the Almighty, for Allah knows and you do not know. Supplicate to Allah to facilitate what is good for you, and reflect upon the words of Allah the Almighty: (And it may be that you dislike a thing which is good for you and that you like a thing which is bad for you. Allah knows but you do not know). Indeed, the believer has what occupies him from becoming attached to other than Allah.