Every time I try to change the paragraph style or the text alignment, it affects all the text in the document, including what i already wrote, and even if I have only a part of it selected. How do i fix that?
Last week these options only changed what you wrote after changing them or what you had selected
I don’t remember changing anything on the settings
That’s what paragraph styles are supposed to do.
If you need to change just part of your document, you can create a new style for that part. You can do that either by in a selected paragraph changing attributes like font and size directly and then in the Sidebar > Styles, clicking on the icon Create New Style from selection, or creating a new style and modifying the attributes by right-clicking the style in the Sidebar and selecting Modify.
More on Styles in the Writer Guide in documentation or in Styles in Writer