Respectfully say that you love online girl as a friend, something along the lines of “I need to set the record straight for both of our sakes and want you to know that I love and care about you as a friend. I appreciate your support and friendship, but lately I feel like I need to step away and do some personal growth offline. You did nothing wrong, this is just something I need to do for myself.” Because from this minimal info, it does sound like you both need to step away from the fantasy that the internet provides and spend more time learning relationships in real life. Avoid talking about the other girl.

it’s not your responsibility to nurture someone’s insecurities. If she’s hurt, she’s hurt, but it’s not going to ruin or end her life. If it “breaks her trust” that’s her responsibility, and she needs to learn on her own what she wants from relationships. You’re not the be all, end all for her like she isn’t for you. At the end of the day, you’re just a person and in a few years you likely will have forgotten each other (speaking from experience).

You guys are still in your teen years, emotions come on strong and fleeting in this age because your brains are still developing and you’re transitioning into adulthood. Your and her feelings are still valid, but it won’t be the end of the world for either of you if someone gets hurt feelings or loses interest. She has years to develop her trust for people, nothing is set in stone at 18/19, so try not to sweat it. It’ll suck in the moment but I guess I am trying to make the point that you both will move on.