My go-to line at work has been 'I'm seeing a councilor about some personal stuff,' it sounds softer than 'therapist.' if people probe further I do the whole 'look, I'll tell you when I'm ready' thing and then I follow it up with a 'hey, by the way, [insert some important sounding question or statement].' if I get called out for changing the subject, I reiterate, 'I'll tell you when I'm ready and not before.'
Also talk to your therapist about why you want to hide it. Sometimes it makes sense and sometimes it's irrational.
Not sure if that helps. 😛