IP fragmentation does not cares about upper protocols, so only first fragment of the fragmented TCP packet has a TCP header. You should buffer fragments by IP ID, not by TCP sequence number and so on…
I would advise to look through some good book devoted TCP/IP protocols before trying to implement something like this. You definitely need better understanding of the networking basics. I would recommend Richard Stevens books (TCP/IP Illustrated), but ofc this is not the only option.