Assuming the valve is 3/4" pipe thread and you are trying to screw in the 3/4" pipe thread end of the brass fitting, then it sounds like you cross-threaded the plastic valve and may need to replace the valve, if that is the case, you can buy one with a flow control and don't need the regulator anyway. I'd re-teflon tape the brass and try to thread it in again, and be forceful in keeping it from cross-threading, sometimes you can overcome the initial bad thread in the plastic with surehanded forceful proper threading of the brass.
