It’s weird that I was thinking about this just yesterday. I concluded there isn’t much good way to make it not sound like an insult, but you could make it less about gender.

Things like “Don’t be a baby.” Or “Don’t be a scaredy cat.” Because it’s often just fear holding someone back that elicits these phrases, and I do feel that encouraging people to push past fears is a good thing overall. There just aren’t any established phrases I could think of that would work the same way without also making the one saying it seem cold.