Unity replaces my saved project with assets samples

Hello! It's my first time using unity to develop a game, but I'm very frustrated because when I finish the terrain and save the changes (through File->Save), later when I open the project all of my work is gone and a sample terrain from one of the asset takes its place, it happened 3 times and each time a different default terrain, I also tried File->save project yet nothing changes.

Idk what to do I really feel frustrated, please help where can I find my project and what is the problem here