How do you keep your mouth shut?

I work in retail, and I swear I talk too much, then I say things that sound weird and then at night I'll think back and be like, omg why did I say that, that sounded so weird. How do you learn to regulate your conversation in the moment, so you don't say anything strange and regret it later? I've been told I don't think before I speak, so this is definitely not in my head.