Wow an ablation at age 10, you are the youngest case I have read about. Did I overlook or did you not say what the ablation was for? Was it tachycardia a high heart rate?
As for can something come back after being cured, I'd say absolutely and I'm not limiting that view to heart rhythm problems.
It sounds like it is time to be checked again by a cardiologist. The HR of 120 at your age should not cause any damage in the near term, but the it is too high to just ignore I think.
Merry Christmas, and hope you find a good answer and cure.
From what i have read the younger you are when you have an ablation the greater your odds of it reoccuring because a younger heart has the ability to heal itself better than an older heart. That said, a rate of 120 does not sound like svt to me. Especially at your young age. My avnrt presented at a rate in the 200s and WPW can be that fast or at least 150 plus. That said, if you are beginning to regrow the accessory pathway it is possible the slow rate is simply due to the fact that the pathway isn't fully restored yet. In any event, I would go and get checked out by your cardiologist so they can see exactly what is going on and if it is of any concern or not. If your svt is coming back you may simply need a second ablation and you should be fine again. But again, a 120 beat could actually be caused by other factors outside the heart like thyroid issues, dehydration or electrolyte imbalances so it is best to go and get checked out by a doctor to see exactly what is going on. Good luck and do keep us posted on how you are doing.