I didn't realize the value of this until after I was done with my undergraduate studies.
Textbooks and lectures will teach you what math is. The concepts, the different proof methods can all come from a book.
The value from an instructor is that they'll give you feedback on the _how_ of math. A halfway decent professor will edit your proof just like an English professor will -- from the level of word choice all the way to the method you constructed and presented your argument. And, just as importantly, they'll tell you when you fucked up and didn't notice.
Textbooks and lectures will teach you what math is. The concepts, the different proof methods can all come from a book.
The value from an instructor is that they'll give you feedback on the _how_ of math. A halfway decent professor will edit your proof just like an English professor will -- from the level of word choice all the way to the method you constructed and presented your argument. And, just as importantly, they'll tell you when you fucked up and didn't notice.