0) You should be using the StringBuilder object to buuilt the line of text.
StringBuilder sb = new StringBuilder();
sb.Append(fixString(blah blah));
sb.Append(fixString(blah blah));
sb.Append(fixString(blah blah));
...
1) You should be write a single line at a time.
2) You still might run into the problem because files cannot be an infinite size. BTW, a string can only be 4gb, max.