You are in control of your emotions, and thoughts.
Try replacing some immediate responses. Tell yourself you’re happy about something, or you’re excited. Eventually your brain will end up believing it.
That’s the same for being down, and that’s why you should never try to diagnose yourself. Saying you’re depressed, or have social anxiety would end up with you being more anxious, and more depressed.
You won’t notice it at first, but over a few months it will have a pretty noticeable effect on your personality.
When I worked in retail, I had a tendency to get real curt with rude customers. When I realized I was just taking the bait, I decided to be super nice even when they would get openly hostile. Some of them would get even crazier and they looked like tools.
If you ask someone to do you a small favour, cognitive dissonance will make them believe that because they did that favour, they therefore must like you.
After all, why would they help someone they disliked?