I had this problem in high school a couple times starting when I was maybe 14 or so. I actually met a girl at a youth retreat I went to and had obsessive thoughts about her over the next year until I met her at the camp again. I had only talked to her for maybe fifteen minutes the first time, she was there again the second time I went back. I somehow worked up the courage to ask her out and we dated for a bit and broke up, never had the problem again after that. I'm not sure if that's good advice or not, just my two cents.