If she's hitting on you and you're not interested, just tell her that you enjoy her company but that you aren't looking for anything more.
One of the problems I see with your fear of hurting others is that you're taking on responsibility for how other people feel. People can feel all kinds of things that we didn't intend for them to feel. Sometimes we change our minds on something we want and someone does get hurt. But they get over it - just like how you got over missing out on the girl who was crushing on you last year.
Love is a risk. Work is a risk. Life is a risk. If we spend our whole lives worrying about reducing risk, then we will attempt little and achieve less.
BTW, did you make an effort to snare that girl when she made her interest known? Or were you too timid? Thing is, it's entirely possible that she just figured you weren't into her and gave up pursuing you. And if this is the case, you can hardly blame her for what she did.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.