Have just bought myself a Fitbit type thing and worn if for 24hours. My heart appears to be going far too fast all the time. Always above 70 and often in the 80s - sometimes high 80s - when I am merely sitting. It shoots up doing the slightest exercise Don’t know how to get it down. Deep breathing and beautiful thoughts don’t appear to help much.
Had my ablation last October and after an initial dreadful time am miles better now. Can even walk 18 holes on my golf course - at a push but am utterly exhausted afterwards of course. Can manage 18 holes with a buggy now no problem. This is such good news.
FitBits - and any other wrist-based heart rate monitors - are notoriously inaccurate. I got rid of mine for the same reason you describe. It would jump 30 or 40 points if I walked across the room. You need a chest strap monitor like a Polar, Garmin or Wahoo. I have a Wahoo and it is much more accurate than my FitBit. Now, when I get up and walk across the room, it blips up 2 or 3 bpm, not 40! My heart anxiety has improved a lot since I stopped with the FitBit.