The forceful beating, I believe, comes from your heart having to work so hard prior to surgery. It is still beating like it has to push very hard to get the blood through. This will settle down after a while as your heart remodels and comes back to a normal size and function. Don't get discouraged, this is all part of the healing process. Your heart is very touchy right now, and rightly so, it has been invaded severely. But it will all improve little by little.
Be very patient with yourself. Allow yourself to feel these raw feelings. Then give yourself a time limit on how this affects you. Also be very patient with your body and its ability to heal itself. It does take time, it has to heal cell by cell. But it will heal and you will feel better.