During your breathe up you may want to try out a technique called Texturised Breathing. This technique is a simple way to control the speed at which you inhale and exhale, which is especially useful during your breathe up when speed of breathing is used to slow the heart rate down.
In The 5 Minute Freediver course I talk about the benefits of extending your exhale breaths and this technique is a great way to control that. Start by slightly pursing your lips when breathing (as if you are blowing a kiss!), bringing them slightly together but still letting airflow through. Allow less restricted airflow on the inhale than on the exhale. This forces the breaths to slow down and helps you maintain even breathing.