Daniel,

First, it is morally wrong for anyone to enter a sexual relationship without being married to each other. If you are a source of temptation for her, you should immediately stop being so even if it means removing yourself completely from her life.

• If she has taken her final vows, it is incumbent on her to fulfill her obligation to God and the community by remaining celibate and chaste — in mind, heart, and body. She would have a special kind of marriage with Jesus.
  
  
• If she is a postulate or testing the waters about becoming a religious sister, it is morally possible for her to change her mind and return to seeking marriage as a state of life.

If I'm not mistaken, something similar happened to Maria von Trapp in the movie, The Sound of Music, however, in this case, she would have to officially leave the religious order before dating anyone.

Pray that God may orient your heart in the right direction; and in the meantime, you must will yourself to stop fantasizing and hoping for any relationship that is immoral.

It may be painful, but doing what is good can often be at first difficult.

Peace,

Paul