When I'm writing sometnhin and when I see that I made a mistake, I can't correct it because my whole sentence is vanishing... it looks like my corrections are eating the rest of the words an it's making me mad

Sounds like you somehow hit the "insert" key. You need to go back into a document and hit the insert key and see how it acts. Then hit the insert key again and see how it acts. This should solve your problem. I've taught word processing since 1983 and this is the only thing that would "eat" up your characters on the screen.

Even easier. Note at the bottom of Microsoft word OVR is black. Click on it. It should go gray and everything will be fine.

PS: It drives you nuts.