my difficulties socializing and helping people

I keep pretending that I know how to socialize, but for example, yesterday I went to buy food for my kitten and I started to get super nervous, not knowing what to say to the girl who was helping me. She couldn't stop talking about her cat, while I just thought "I need to go... am I forcing my smile too much?"

Generally I go to buy things with everything in mind, and when things don't turn out as I wanted or expected, I start to react strangely and feel super nervous, not knowing what to do or say.. TT

On top of that, I can't even start a nice conversation, after "Hi, how are you? :D" I don't really know what to say anymore, but I still try to have a nice conversation 💔