Why would a girl lead on? I emphasize this question with many factors (there's a lot to it though)?

I'm a young adult. I waited 6 months to ask a girl out after she finished college for the spring semester, I asked her out in the summer.
She says yes to me and agrees to set up a specific time next time I see her at work,
I didn't see her till two months later at the job, ( I guess she changed her hours?) and I asked her if she was still interested.
"Ask me later."
Why? What does that mean? Why would you do that?
Does it mean she's playing hard to get? Can nice girls be like that? She struck me as a really genuine, patient person, and I don't want to really acknowledge it like that.
After some time passed again , I asked her a third time and she just kept telling me how busy she was.
I know what you're going to say, but I'd like you to answer the question first for some closure.
I feel really against my nature to ask anyone out every again. After what I found what I felt the most incredible person ever and the most beautiful heart and character, I became completely distraught, but I can't drop my job for this.
I see her paying attention to me often, but she hasn't once agreed to meet with me outside the job, and never gave me her number ( Im against social networks by the way).
How can I feel good about ignoring her ( i guess this is going off the question, If anyone cares to respond I'll just put more stuff here)
Should I ever ask again
If before I leave my job she approached me or anything or something should I try it?
Should I seriously stop trying to think about it after all this time I've spent thinking about it and all the while I see her at work.
How do you feel good about something like this that you have the greatest craving or desire for in your life, where you feel you couldn't be satisfied or filled in any other beautifully, enriched way. In the mind , this is such a sickness to me and I don't know what to say or think anymore. I might be crazy, if anyone can say anything , I' could say more but limited
Why would a girl lead on? I emphasize this question with many factors (there's a lot to it though)?
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