I Feel Used

I feel like people take advantage of my caring personality. For example: I saw a girl sitting down alone on my first day of high school. It was her first day too and I befriended her. I talked to her, I sat with her and I was really nice to her. But all of a sudden she starting sitting with some other girls and completely discarded me, knowing I had no one else to sit with. It's like she thought 'some cooler girls are over there, I'll just dump the only person who has been kind to me from the start and go sit with them.'
Every time I have been 'friends' with someone, they only keep me around for so long, and now I realise they're only nice to me when they want something from me.
I know it's probably selfish to think like this, but I just really want someone to praise or acknowledge the things I do for people. I know I am a good person and I'm not trying to be vain, but I do wish that someone would mirror me and be kind to me for a change...