You're in good company. Palpitations freak a lot of people out, and I've been there many times. OCDman mentioned no caffeine, and he's right. Stay away from energy drinks, lots of coffee, etc.
Keep in mind that at age 19, the odds of your having a heart issue are incredibly small. And since the EKG and bloodwork came back normal, it's almost certain your heart is fine. And the holter monitor will probably come back normal too, except for some PVC's and/or PAC's which are normal and benign. Try some stress-management techniques like deep breathing, yoga, meditation, guided imagery, etc.
Finally, try not to hyper-focus on your heart. It knows what it's doing. Just let it do it's thing and it will take care of you. :-)
You're probably fine. The ekg will likely tell you that but IF there is a problem it will pick that up too. Had similar experiences around your age. Freaked me out. Đr said it was normal. Stress causes them and they stressed me out so vicious circle. No caffeine for a bit, exercise and relax. You're song the right things by getting checked. Palpatations happen to everyone. I still get them some days and they still unnerve me. Try coughing during one. Might stop it. Good luck. And relax :)