Firstly, Doug's question about the platform should not be dismissed ("However the most important thing for me is to build a regex") as it is really central to our being able to answer your question. Different regex variants that run on different platforms each have different capabilities and ways to approach the same problem.
Because you are using the .NET regex, you can use something like
^fstring\r\n((?!^fstring)([^\r\n]*)\r\n)+
with the 'multiline' option set on (and possibly the 'ignore case' option on if that is applicable). Depending on any processing you may have done on the text before you use the regex, the '\r's in the pattern may not be needed - they were on the .NET based regex tester I used with your example text that was simply cut-and-pasted.
This assumes that the "fstring" text is always at the start of a line and that there is always at least one line after that. Further it assumes that each line is to be captured in its entirety.
Each match will begin with the "fstring" text and will match everything up to but not including the next "fstring". If you look at the captures of match group #2 you will find each of the lines that follow the matched "fstring".
Susan