even though its not exactly the same, i did something similar. from my junior high to high school transition, i dumped my first love on the mere fact that i was going to a different high school..
..i sed the same ol' b.s. to her like your lady did, but the truth was that i wanted something different and new. i was extremely selfish at the time and i feel bad about it to this day. i don't regret it, but i feel guilty.
with your lady, she might be feeling the same thing along with a little shame. it's not common that an 18 years old girl would go with someone younger in high school. you should feel blessed and feel like a pimp.. i'm sorry bout your losses, but i guarantee you'll get over it soon. :thumbup: