\b will match word boundaries, so any transition from a word character ([a-zA-Z0-9_]) to a non-word character.
Since it sounds like you do not want to match if / follows your whole word, you are going to need a different boundary check. Something like the following should work:
string TextToFind = @"(?
This will cause the match to fail if the character before your word is not whitespace, or if the character after your word is not whitespace. Note that I used negative lookbehind/lookahead here instead of (?<=\s) and (?=\s) so that you will still match if your word is at the beginning or end of the string.
