Embarassed to speak fluent English

So I have been bilingual since my first words came out speaking both English and Greek. I live in Greece and travel to the UK very frequently. When in the UK, I have absolutely no problem speaking in English or Greek. When in Greece however, I am simply too embarrassed to speak English in front of Greek people.

I have to give a small speech in front of 50 Greek people soon in English… And it terrifies me…

Has anyone every had this issue? If so, did you get over it and how?

Many thanks in advance for any feedback/tips you can give me.