Your friend is frustrated because they spent time, energy, and possibly money making plans to spend time with you, then you told them the effort they made was wasted because you'd rather do something more important to you. They may feel like you don't value their friendship.

A better way to handle this is to approach your friend with the decision and ask them how they feel. Instead of telling your friend you won't go on the trip with them, which puts them in a powerless position, talk to them about the panel at the tech conference. Explain the opportunity, what it means to you, and then bring up the fact that it conflicts with your planned trip. Ask them if they would be okay with taking the trip another time, and apologize for the short notice. Your friend would still be disappointed in this scenario, but the decision to cancel the trip is mutual. This is more respectful of your friend's feelings and time.

The way forward is to apologize for not considering their feelings. Acknowledge that you should have asked them first before cancelling your plans and that you made a mistake by not thinking about how the trip was important to them. You have ADHD, which can lead to impulse actions and sometimes you act without thinking all the way through. Ask them if they'd like to take the trip another time, or offer to spend time with them in a different way.