That's what your fantasy was
This was your fantasy
This all was your fantasy
What is the difference between these three sentences? To me, being a non-native speaker, they sound alike. But I know there have to be some differences between them.
Some differences for me:
(Note: the third one should be 'this was all your fantasy')
They are basically the same, but have slight differences in emphasis.
The first one emphasizes 'that'. You could use it when you want to focus attention on what the fantasy was. E.g., if the fantasy was surprising in some way, or to sum up at the end of a paragraph.
The second one is a plain statement. 'That' has been changed to 'this', but the sentence is also fine with 'that'. Changing it to 'this' adds immediacy.
The third one sounds more natural as 'This was all your fantasy'. By adding 'all' you are emphasizing that it was entirely fantasy, with no basis in reality.
But they all mean pretty much the same thing.