Make an appointment to see your GP. It is possible to get help with anxiety.
What is your employer’s ‘Lone Worker Policy’? Read it carefully and check the procedures that must be followed in case of any incident, make sure that those procedures are available to you, rehearse what you would do in the case of an incident. If necessary rehearse until you can do it almost in your sleep, that will help to build your confidence. Try to remember that your employer has confidence in your ability, or you wouldn’t have been given the job.
However, rather than ringing your manager, if you are having a panic attack, is it possible to ring a family member/friend? Your manager’s role is to be there for you in case of a work related incident, rather than to help you to deal with your own issues.