I'm sure you will get better advice but I can tell you what I did :) Vida didn't have a whoa either when I first got her. The only way she would stop or even slow down was a one rein stop. Not ideal when your getting ready to cross a busy road
I spent the day with her in the paddock just giving her the cues to whoa as she approached the fence. She had to stop or run into the fence. Use a corner if a straight fence won't work. Say "whoa" every time and use your leg and hands. I'm not sure of the right order, but I go verbal first then hands. Give lots of praise when he stops. Rub him say good boy and do it again and again. She stops at the verbal "whoa" now.
I want to add that I do a stop-back thing when we are warming up. Make it all one motion stop- back. He has to learn the stop first though
It sounds like you may have some biting issues too. Or you may be pulling too hard.