Impulse Spending at Comic Con

As well as having ADHD (finally got diagnosed!!) I am a huge nerd (pretty sure it’s AuDHD). Me and my partner go to comic cons 1-2 times a year, and every time I spend WAY over my budget. I’m so good at justifying the spend to myself as something useful, or something rare, and I don’t know how to get out of the pattern.

We’ve got a convention this weekend, and my bank is currently on about -£700. I’m so worried about what I’m going to spend. Anyone going through anything similar? Any good advice?