I can't shut of the hot water completely
Go back and once again turn off the water main valve where cold water enters your house. Then go to various sinks and bath areas and turn on the faucets there. These will allow the water from higher level piping to drain out and break any potential siphon action that may have been occurring in your shower where you were trying to do the repair.

There are a number of reasons why a replacement of part of that hot water valve insert may not be working:


The wrong style of insert is being used.
The insert may be too thin.
There is debris in the faucet body that prevents the faucet stem from being turned completely in to bottom out the insert.
There a crack or corrosion damage of the seat part of the valve that prevents the new insert from even working.