Should I fast with the neighboring Islamic countries or break my fast with my own country, knowing that my country relies on astronomical calculations and has declared today, Thursday, as the first day of Eid, thus differing from the rest of the Islamic countries?

You are not permitted to break your fast except by sighting the crescent moon of Shawwal or by completing thirty days of Ramadan. Relying on astronomical calculations is not permissible. If the state relies on moon sighting, then all people are obliged to break their fast on the Eid day if the state announces it. However, if the state relies on astronomical calculations, then its declaration is not to be considered.