Recommend you get an appointment with your GP, and or a referral to an orthopedist. Since seeing my GP I've been getting some relief by splinting and doing passive stretches (the tendon gliding stretches actually exacerbated the problem). I have an appointment with an orthopedist this week. Based on the progress I'm seeing I'm hoping that I can treat this without shots or surgery.
For fly fishing I found that a simple splint on the affected finger only (ring finger for me) was helpful. I definitely had to loosen up my grip and concentrate on the upper and lower leverage points where the hand addresses the cork. I think my casting actually benefited from this change.
That's just my experience so far. Seems like it's a bit different for everyone. I hope you find relief.
@Bagman
BTW: I get a bit freaked out when the finger locks into the trigger position -- I'm afraid that something is gonna tear or pull... freaky stuff.