Alright, problem is solved aswell

Basically I wrote a loop that goes 10 to 20 times and
reads 4000 letters (could be 10000 in newer versions ) and attaches it to the end of my String.

For some reason, if the end of the document is reached but it still is looping, it just kept on attaching the last read portion (I think that is because the read/write pointer goes all error when it is positioned too far).
I solved this by just comparing the current read out portion with the last one, and should they be the same, it will just not attach it to my String .

It actually works really well and I have tested it with up to 80.000 letters (an 80kB .txt).

This only works with light C, since light C Strings are unlimited