I know this is not super-easy, but there are a few things you can do to make it easier and more comfortable for you.
1) be prepared. Think about what you want to tell them before, maybe even write it down so you don’t forget any point that’s important to you.
2) it’s best to tell them when they and you are rather relaxed and there are not many other things they have to take care of in that moment.
3) they might ask you questions or be confused. you can tell them it’s hard for you to talk about this and that you might not be comfortable with answering all of their questions (that’s perfectly fine if you don’t want to answer everything, by the way!). you can also say before you tell them about your anxiety that you want them to listen and think before they react. Sponaneous reactions can be hurtful and I guess you don’t really need that. Most parents need some time to get used to the thought of their child having anxiety or any other mental illness, but they hopefully will be supportive then!
4) you can also talk to your doctor first and ask them to help you to talk with your parents. or a school counsellor. or maybe if it’s easier for you to tell an uncle / aunt first, or older siblings, or a friend, you can always do that and ask them to accompany you and help you with the talk.
I hope this helps a bit! :)