Why did I have such a hard time properly creaming butter and sugar?

I know what properly creamed butter and sugar is supposed to look like, but I don't understand why I had such a hard time achieving this. I was making cupcakes for a friend and the first step is of course, to cream the butter and sugar until light and fluffy. In the end result you should not be able to feel the sugar granules when it is properly creamed. My butter was at room temperature (not melted) and I spent a good 20 minutes (no exaggeration) beating the two together on high speed with my Kitchenaid mixer before I finally gave up. The butter was beat for so long that it actually started to melt instead of cream. It looked a lot like wet sand. I do live in Florida and it is hot out right now but my a/c is set at 74 degrees F. I don't know what happened! I even tried putting the mixture in the freezer for a bit and starting over, but that didn't help at all. If you have any explanation or advice, please let me know. This has me very frustrated! Thank you!! (: