Are you open to non-VBA solution?

How about an old fashioned command line solution?

The easiest way to work with text files of this size is to use the DOS
Find command. The drill here is to go to a command prompt and then
'CD' into the folder that holds your big text file.

Then type: find "B/M" filename.txt >fname2.txt {enter}

This will filter the contents of the big file  and look for any line
that contains the substring 'B/M'. It will then copy any lines that
contain this text and output them to the screen. The '>fname2.txt'
redirects the screen output into the file named fname2.txt.

Once the new file is created that just contains the 'B/M' lines...
This should be a significant subset of the big file and you should be
able to open this in Excel without having to jump through a bunch of
hoops.