A good way to overcome something like this is simply to research about said clothes. What clothes do you like? What style is it usually categorized as? What material do you find comforting? What sizes are you? Sometimes just knowing the answers to these questions can make you walk in confident.
When a worker asks if you need help, you can tell them exactly what you're looking for in utmost detail. I even find that if you shop intuitively like this, you get better service and it doesn't become a task of getting clothes and getting out.
And by better service, I mean laid-back service. After a confrontation like that you can just be like. b--ch please give me space. And they will moonwalk 400 paces away from you. You can still tell them you're just looking from the beginning. But it's good to be confident rather spontaneous and uneasy. At least , that's not how I would want to feel, you know.