Why do I get jealous over a girl even though I don't want to be with her at all?

 The correct phrase is "All this is correct."

"All this" refers to a singular noun, even though it implies multiple things. In the example sentence, "all this" refers to all of the information that has been presented.

"All these" refers to multiple plural nouns. For example, you might say "All these answers are correct" if you were referring to multiple answers to a question.

Here is another example:

All this food is delicious. (refers to all of the food on the table)

All these foods are delicious. (refers to multiple different types of food)

I hope this helps!