My dental hygienist gives me a tooth brush at the end of every visit. With regular tooth cleaning visits I seldom purchase any. They are stiff the first time or two that I use them but soften up quickly.
I would suggest that you soak the new brushes in water for a day or so before using them. If that does not soften them enough, add a bit of soap to the water and rinse them well before use.
And if you don't have a tooth cleaning appointment at least every six months, I suggest that you start doing that.
Paul A.
Make it fit.
You can't win and there is a penalty for trying!