I had the same problem as the OP when I started seeing T. I'd have an agenda, then get there and find more important things to discuss, then leave being annoyed at myself that I didn't get to address the things I REALLY wanted to discuss. I learned to work around that though.

Since T doesn't want me to bring my phone in session, I write down things I want to talk about and bring them in. Sometimes in the car on the way to session, I'll keep thinking about the things I want to talk about so when I see T, I"ll start with that.