It's impossible to fit the cam follower the wrong way as it only fits one way.
Have you got different internals in (Autotech, KMD etc)? as this is a problem associated with that type of piston design.
Which valve are you after removing? The schraeder type valve on the side (the bit that's like a tyre valve)? If you have fooked the head of that up you can knock a smaller sized socket on it to get it off. You can also buy that valve on its own for about £3 from a dealer (I did the same thing)
If it's the actual internals you want to remove then the process is a bit more involved and requires the pump to be removed.
If you just have a standard OEM pump then as has been mentioned maybe your internals or seal has gone (i'd be surprised though if it's standard).
A new OEM pump costs circa £180 from a stealer and if you have fitted the cam follower yourself you should have no problem fitting the new pump yourself.