Ah I remember learning that section. It was a royal pain at the time, but my teacher recommended playing the phrase with a metronome on all sorts of varied tempos without the trill. Then, slowly work on it as a measured trill. As someone else mentioned, there is no definite way of playing trills in 18th century music. I personally prefer starting that one on the note. This will get easier with lots of slow practice. There really isn't a short cut, I'm afraid. Keep at it and it will eventually fall into place