Welcome to the world of dating 21 year old women.
They have an "out of sight, out of mind" attitude toward all men at that age.
Their selfishness is off the charts, but that is what God decided was optimal in his quest to procreate humanity.
Things I did to get my last one back:
1) Offer a date and when she stalls, retract the offer (this is when the trouble started, but it was handled correctly).
2) Bump into her but avoid her (stay on her mind).
3) Make her miss you (go MIA for 1-3 weeks).
4) Be ready to walk forever. At this point the ball is in her court - she must initiate.
5) Assuming she does initiate (in my case she sent sexy photos of her body), wait 1-2 days to reply (ideally you are too busy to see her message).
6) During your time without her, determine what really makes her special to you, and remind her about this when you do reply. This hits her with a 1-2 punch of elation that you are back, and making her feel beautiful (which was why she liked you to begin with).
7) Do NOT mention the fact she avoided you. You're too happy with other women and overwhelmed by your desire for her.
8) Have lost weight in the meantime.
9) Have got a tan in the meantime.
10) Have dated other women and have some good stories in the meantime.
11) Have got a new job and gone on a 1000 mile journey to a new city in the meantime (to make her interested, be interesting).
12) Meet her again and let her see the new you.
13) Tell her some great stories of your time away.
14) Tell her none of the other girls compared to her.
15) Kiss her and make it a good one (this can be the highlight of the entire courtship - the makeup kiss).