It's completely normal to be nervous about the psychology appointment, especially when it's one of the last steps and so much feels like it's riding on it.
In my experience, the goal isn't to trick you or look for reasons to deny you. They're trying to make sure you understand the surgery, have realistic expectations, and identify any areas where you might need additional support for long-term success. The best thing you can do is be honest and be yourself.
You're on your last appointment! You've already made it through all the previous requirements, which suggests that your team believes you're a good candidate and is continuing to move you forward through the process.
For both of my bariatric surgeries, I was required to write personal letters that were reviewed by my surgeon and psychologist as part of the approval process. If you think it would be helpful, I'd be happy to share what I wrote so you can get a sense of the kinds of things I discussed and what the process looked like for me.
Try not to put too much pressure on yourself to say the "perfect" thing. This isn't a test that you're expected to ace, it's a conversation.