You already stated the problem: you get tense and she picks up on it. Rather than working on what to do different with her to get her attention, focus on deep breaths, relaxing, etc. You'll be amazed at how different things will be once YOU'RE confident. Like you said, she's great out on the streets - most likely because you're confident.

Dogs are fabulous readers of body language. When she sees you tense up, she doesn't know what to do. She's looking at you to be collected & in charge like you usually are. When you're showing signs of stress, she basically runs ahead because she knows you're no longer in charge.

An easy fix..... but these things are always easier said than done