How do I reply to this?

If you really trust her, then you should tell her- as long as you feel comfortable. Doing it over Facebook could be odd, so maybe you could just say something like "My summer's been a little hectic. Can I tell you something important?" If she says yes, then when you reply tell her that you would rather tell her in person. After you've confided in her, try to break the tension by asking her to teach you how to ride a bike. That may just be the escape you need from a rather stressful life.