my 00 ws6 grinds into 3rd gear , i change the trans oil and this aluminum pc came out and a lot of chew up hard black plastic .

now as im driving i put into third gear and it grinds, then after trying and trying , sometimes it goes on third , and then it grinds on 4th , and try again an again and it goes on 4th , but most of the time its grinds

what is this aluminum pc i found and all the plastic???? how does it affect d trasnmission if i keep forcing it with this pc broken , because sometimes it shifts ??????is it easy to repair????and how much im i looking to spend in fixing it ???