Got played so hard

So a couple weeks ago I met a guy. I guess met isn't the proper word because I've know him but we started talking. He broke up with his girlfriend on a Monday and began talking to me on a Wednesday. I knew from the beginning I was probably his rebound and that he obviously wasn't over her but I continued talking to him. He even asked my sister for my number. We flirted back and forth and last night I found out him and her got back together. I feel so stupid and worthless.