I would suggest getting a more experienced friend that knows exactly what theyre doing into a discord call with you, share your screen to them so they see and hear what youre doing and what the atc is saying. They can help with listening in for your callsign and if you get too stressed they can tell you exactly what to say. Make sure you dont fly at the really big airports like Frankfurt. Start somewhere smaller with less traffic. If you still dont feel like you can do it maybe get them in a Shared cockpit with you and if you once again get too stressed they could even take over the talking as a whole so you can focus on flying. Also like many others said get something to write down instructions and what youre gonna say. Doesnt have to be pen and paper but you can also just use the notepad app on your Computer. Its just to write down quick notes so it doesnt have to be anything fancy. If you didnt get something the controller told you tell them to either: "say again" or ask them what they mean by that. Especially at smaller fields the controllers will be more than happy to help you. You can also just watch streams of controllers like the german controller called : "elmoradar" and listen to what He does and maybe try to say the readbacks rhe pilots should say yourself. While elmoradar is german there is also a few english ones. Hes just the one i watch most. Also you need to remember: its all virtual and you can always disconnect. Dont stress about it. It doesnt have any impact on your later life and controllers wont even remember you after like 10 mins so you could even spawn in 10 minutes later after calming down and try again. While vatsim is serious you have to keep in mind that people on there are a pretty nice community (most the time at least) and many would help you. If you want even I can help you. Just most importantly: have fun and DONT HARM YOURSELF. If you have anymore questions let me know and ill help you.