I used to be like you. It doesn’t do you any good to pretend that you don’t stutter by avoiding talking about it. Everyone already knows and there’s absolutely no shame in talking about it. When you accept stuttering, the pressure to not stutter will mostly go away, making it easier to speak without stuttering. As far as your shitty co-workers, there’s not much you can do other than confronting them and explaining that you stutter and that’s why you’re so quiet, it’s just difficult to speak. Or you need to speak to your manager.