Hi Evie, I found that with controlled breathing to get heart rate down it helps to find a rhythm that feels as natural as possible. Could you perhaps experiment with the number of in & out breaths & pause length, so that the exercise feels more comfortable than forced? If we end up doing one or other stage for too long I think this taxes the body.
I also found that once I got the breathing rhythm right, I still had to continue the exercise for around 20 mins at a time in order to properly calm my system. Impractical in some situations of course, but during the night if palpitations & panic strike, there is more opportunity to really concentrate on the exercise & see it through.