Half of water when shower is on comes out of the spigot- how can I fix this ?

First, you have to tell us what kind of faucet you have: Do you have 3 handle faucet ( hot, cold and diverter ) or do you have to push a knob on top of the spout to transfer water from spout to shower ?

No it is a symmons anti scald ? Dial with rotator on the bottom

Which one is yours Tub/Shower Systems Your diverter is only partially closing but it can be rebuilt