I'm losing her- how can I put a stop to it?

I'm a guy but I wanted to see if you could help me out with a bit of a situation I'm in. I met this girl last year and we've been best friends since pretty much the day we met. We're such similar people and completely click yet we're so different at the same time it's pretty amazing. We both agreed that we don't want to get into a relationship and ruin what we have because it's so amazing.

Recently, however, she made out with a guy on a night out which was fine but then it started making me feel really horrible. She's also been going on dates recently which makes me feel that I'm going to lose her. I don't really understand what to do because if I tell her how I feel it has the potential to ruin everything. Going a bit crazy because she means so much to me... HELP!