Apparently I have a mean look. What to do?

Recently, I had someone tell me upfront that I look miserable and have a "resting bitch face." It explains a lot since I've gone throughout life having people give me dirty looks and generally being cold towards me. When someone is genuinely kind to me, it takes me by surprise and I don’t know how to react in the moment. I always thought I was ugly, possibly on the spectrum and/or giving off a repellent vibe. What can I do?