How do I rebuild attraction with an ex girlfriend if we're friends right now?

huck13
Background: we started dating in feb of 010, broke up in feb of 011, so it's been about 13 months since the breakup. we were each others' first loves. breakup was mostly my fault, she was the one who ended it, and I was pretty depressed and messed up for a while (stereotypical breakup story).

i was blocked from April until August, and even after she unblocked me, she was always pretty rude and cold toward me. things finally changed for the better about 2 months ago in January, when she emailed me an apology for the way she had been treating me and offered to be friends.

things have been decent between the two of us, she drives me to school in the morning on Mondays and she always responds quickly to my texts and chats. we even hung out for a little over an hour during mid winter break about 3 weeks ago.

she was flirty and giggly during our hangout, but in the mornings when she drives me she's pretty quiet and doesn't really initiate conversation. her behavior toward me is kinda bipolar at times, she either looks at me a lot or tries to avoid eye contact. she's either flirty and giggly or quiet and reserved. I guess this is the part that's kinda confusing me.

my question is, is there a way to rebuild that chemistry we once had? she's EXTREMELY busy (not just high school busy, she's literally gone practicing archery every day because she won some junior world title over the summer), so spending a lot of time with her isn't an option. that one hangout we had a few weeks ago was a really lucky score for my part, she literally had about 3 hours of free time for that entire week long break.

also many of you might tell me that its been a while since the breakup so I should find someone else, but I still feel like she's the one I love most. there's another girl that I'm super close with now and we're Facebook siblings and talk quite a bit. some friends even started teasing us and calling us a couple. I love her very much, but somehow I feel like its more of a sibling love, while my love toward my ex is romantic.

just an add-on: my friends were openly teasing me on Facebook yesterday about being in a relationship with this girl...my ex probably saw this Because she was online when all this happened. she was a bit more talkative this morning when she drove me to school but not significantly.

any advice on how to rekindle old sparks through friendship?
How do I rebuild attraction with an ex girlfriend if we're friends right now?
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