SilverWook said:

I wonder at what point he changed his mind and decided he wanted to make a third trilogy?

As they old saying goes from All The Presidents Men, "Follow the money!"

Lucas said an in interview on Charlie Rose in 2004 that his company was having financial problems in the early 90's. His shareholders went to him and did a presentation on the financial effects of making the PT. He then said (paraphrasing), "I could have walked away from SW right then and there, and would be free to make whatever movie I want to make, but I would always be reliant on the studio system. The financial benefits of making the PT is that it would make my company independent and free from the Studio System forever."

Lucas was asked about his plan to make 9 movies again right before Episode 1 and he answered, "Ha-Ha, let me make this trilogy first!" He could have easily said right then and there that the Saga is 6 movies.

Then I believe Lucas was fed up with everything after the PT, and didn't want any part of SW again in the form of movies. But again: Follow the money!

There was still a demand for SW (despite the PT), so he was either going to make Episode 7 and then sell, or just sell altogether and let someone else do the ST. There is just too much money to made on SW to stop milking this cow, Lucas knows it, Disney knows it, we all know it.