I feel much more confident at work in terms of both my English and the ability to negotiate with my counterparts.

counterparts 是 counterpart 的复数形式。counter- 表示"对应的、相反的"，part 表示"部分、角色"。counterpart 指在另一系统中具有相同或对应职能的人或事物，这里指"谈判对手"或"业务对接方"。