Is it forbidden to continue speaking with a girl whom I love and wish to marry in the future, but am unable to do so now, within the limits of general topics and refraining from sweet talk and impermissible speech?

You must immediately sever this relationship and repent to Allah. Sincere repentance necessitates avoiding the causes of sin. Therefore, beware of calling or messaging this woman. Do not let Satan deceive you with the pretext of pitying her or limiting your communication to permissible speech; these are satanic tricks that lead to corruption. So, close every door to temptation and cut off all ties with her, and remove any attachment to her from your heart. This will be easy if you seek Allah's help and strive against your own self.