I agree with Leramy and the post below hers, the problem isn’t the bit totally, you need to go back to basics, you can’t ride tack or rely on it. You need to have proper communication with the horse so he respects you enough to stop when you ask him to either with your seat or with your voice. When my young mare started undersaddle her whoa was less than ideal, I got on her in a round pen, bareback and bridle less with just a rope halter and the lead on only one side, we worked for a while together until she realized that I want her to whoa when I ask with my seat and my voice, I had NO grip on the lead rope what so ever. I now have conifidnece if we are out on the cross country course and my rein breaks (heaven forbid!) I will be able to ask her to whoa and she will gladly do it. Best of luck to you two! I’m sure you will do great!!