Tub Shower Units I Have A Tub/shower Unit And The Knob I Pull To Use The Shower On The Tub Faucet Is Stuck. How To Help?

I have a tub/shower unit and the knob I pull to use the shower on the tub faucet is stuck. How to help? - tub shower units

Used to work and it was hard to pull and push. Now he is hanging and I'm afraid something is broken, if I May shoot harder. I can imagine that would replace the valve, but was waiting for a solution. Thanks