My personality is awkward and brittle

Whenever I meet new people I feel myself become stiff and my mind completely empties. Whenever I’m comfortable with people I know I can think of things to talk about easily without much thinking. This matters so much to me because I’ve failed just about every single first impression I’ve had with all of my new coworkers as a lifeguard in a big gym facility. I also need some help with responses. What am I supposed to say other than “wow” or “that’s so cool” when someone says something that happened to them was really interesting. I can only bounce back with them for so long.