Thinking back to girls I knew in high school who wanted to be chaste but succumbed to sexual temptation, I think they had a few things in common.
1. They were very emotionally needy girls. They lacked male attention in their lives, or they wanted the love of a boyfriend to make them feel whole. They lacked a sense of accomplishment to create a sense of self-esteem.
2. They dated boys (or men!) who were not committed to abstinence before marriage. That should be a "deal-breaker", as Dr. Phil would say. I know that many girls have this inner fear that if they insist on waiting until marriage, they'll restrict the pool of potential candidates so much (because boys don't want chastity, so it is though) or that they might pass up a guy who's otherwise wonderful. I've been there. I think the answer to that is to develop a sense of personal integrity to want to keep to one's own principles because it's important to you, no matter who else doesn't think it's right. High school can make you feel like no wants to be chaste before marriage, but when you come to adulthood and discover world, you'll discover there's a significant minority of people who want to be chaste. But even if you never discover that world, personal integrity must matter more than companionship. Self-esteem has to come from the inside first.