How to walk less stiff?

I've had this problem since middle school, people would say things like "you walk like a high class whore" "you walk like your constipated" "Your butt doesn't move when you walk" "you walk stupid". Now in high my ex crush's (look at previous questions if you don't understand) friend commented on my walk and said "she walks like she has a stick up her ass", and I still think about it because I thought that I stopped walking that way, it's also one of my insecurities/flaws. When people comment on it, it makes me tense up more. It doesn't help when I try to relax in public either since I keep thinking about how to walk correctly. I don't understand why some people are so focused on my walk :/
How to walk less stiff?
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