I've had the same symptoms since I was about your age. I've learned how to deal with it now to the extent that I can usually ride pain free all day. For me, I'm convinced my back muscles carry tension in them without me usually being aware of it. Riding hard makes it worse. But that's frustrating because I want to be able to do just that.

My most important thing has been to pay close attention to the feeling and become aware of it before it becomes pain. Figure out how to make that feeling go away now. Concentrate on relaxing your back. Try to drop your belly low and make that your default position.

I think yoga helps both physically and by improving awareness. Keep a full range of motion so your body can ease into the right riding posture. Exercise helps build stamina. But also manage the tension and don't let it become self feeding.

Don't pull on the bars a lot. Do that only for little bursts.