Do I really have IST?

Hey everyone! I have been diagnosed with IST a few years ago by the results of my ECG, but they told me it is probably because I was nervous. This week I have been seeing a new cardiologist and this time I got another ECG, an echodiagram and a holter monitor test for a day. I also have a false tendon since I was born, but my cardiologist said it’s fine by the results of my echocardiogram.

The results of the Holter test showed that I have an average of 77 bpm, during the day it’s 82 and during the night it’s 66. But when researching I’m seeing that average below 90 is normal, so I’m confused? I trust my doctor, but I don’t trust myself if that makes sense. I am 20 years old, male. The most notable parts of the holter test are: during the day, I had maximum bpm of 137, 106, 82 and when I was climbing stairs fast I had bpm of 172. However, the average during these hours is 99, 81, 74 and 125 respectively. What I am more confused by is that during my sleep the average heart rate was completely normal - ranging from 55 to 65. I was prescribed ivabradine for 2 months one 5 mg tablet every morning.