How do I practise a difficult middle passage on piano when I keep restarting from the opening?

My teacher asked me to start at bar nineteen. I put my hands on the keys and couldn't find the first chord. From the beginning, I can reach that bar without stopping.

I'm learning piano as an adult. I enjoy the opening of this piece and play it repeatedly. When the middle falls apart, I go back to the start because that feels like playing music again. In a twenty-minute practice session, the opening gets several turns and the difficult passage gets a few anxious attempts.

The teacher has told me to work on the middle separately. I agree with the advice, but I haven't asked how to enter that section when the earlier bars normally carry me into it. I feel foolish turning a practice instruction into another question.

What should I ask her to demonstrate at the next lesson? I'd like a small task I can actually begin, while keeping some time to play the part I enjoy. At the moment my very confident opening is escorting me to the same unfinished place every evening.