To be fair, I know plenty of decent girls under that age who would never do that to some one. Plenty of girls under that age who don't go out looking for sex when their single or depressed.
But as stated before, no one should feel dissapointed over a one night stand. The whole night was a lie? I don't know how you can feel anyone you just barely met some days before and have sex without being anything more than friends who met recently can be tooken seriously. Its not that I don't believe it can happen that way, but you should have seen that she was depressed and looking to have a good time. You were nice to her and she liked you. I bet she doesn't like you that much anymore since you posted it on the internet. What is said on massassi, stays on massasi!
If you want to find someone serious, just continue with your life, have more one night stands, hanging out, studyin etc. Eventually you'll meet someone good.
Could take a while though!
Nothing to see here, move along.