Looks like a normal standard reaction to being TFTGed.
Here's a story that I found that is also post TFTG but it has the mental process in it. First he starts out getting used to his new female pony form then in the 3rd chapter the mental process starts as his mind is being invaded with pony throughts and memories. It starts to get so bad that he can't even
say that he used to be human or even a guy. Then it becomes even worse as his mind deteriorates into Happy-Go-Lucky-Pony-That-Wants-To-Have-Fun-All-The-Time (He doesn't turn into Pinkie Pie but he starts emulating her) after his encounter with a human that could've helped him went bad.
http://www.fimfiction.net/story/435/