From the opening phrase in your question I get the impression you regard yourself as not long on experience with whistle in sessions. Oddly enough, it does take practice to hear yourself properly in a good session where everyone's cracking on, even when you're not surrounded by other whistles crowding that particular aural space.

The three posts above are all correct but you might better determine how much you are contributing (as well as the whistle itself) to the 'lack of control' if you positioned yourself, for only a few bars even, at the edge of the main sound. Sometimes even just playing with your head turned away, for the critical bars, can help.

Cheers, Rowan