[Bramblecot]

Lovely photo, lovely dog, and lovely that you recognise his sensitivity and are prepared to work with it. 

My first search dog came to me at 2 1/2 years old from a Field Trial where he had 'misbehaved'.  He was a wreck - terrified, beaten, broken physically and mentally.  It took years to turn him into a confident working dog - his natural ability was superb but bad handling ruined him.

Re the training field.  IMO dogs def. associate certain training with specific places ( I could go on for ages, pm if you want) so keep using that flat field for nice successful exercises before you try something he struggles to achieve.

You are right at the top of the league, it's a case of fine tuning now.  Be proud of what you have achieved with him, well done.