Disciplinary Issues: Prolems with Boss Also the Owner

my boss is also also the owner and i have worked there for the last 12years 13 in october .........the first 7 years were fine but i had injury to me arm which i done at work never claimed but had to have 15 weeks off work came back to work and as i couldnt do heavy lifting was ok by boss he understood but after 6 months got 1st letter of many to say wasnt happy with my work had letter to say would take further if didnt improve my standard of work 6months later had meeting and again same thing... 6 months later again samething and this has carried on until now which today got another letter stating not happy with my standard of work in between each letter (6 months) have had no contact with him to say if my work has improved or not. all the other team members all do far less than me but never seem to get a meeting or letter just verbal telling off feel like he has singled me out just had 8 weeks off work cos i broke me leg one month back at job and i get letter ............dont know what to do but i am not quiting my job but it is proving to be too stressful wondering what to do to keep him off my back

Disciplinary Issues: Prolems with Boss Also the Owner