Yeah it's tough to not be able to make deadlines, but you definitely at least learned things to take away from it! Often it's what you feel like you failed at that teaches you the most.
I can't do art and primarily am a programmer, and I think your best bet for a game jam is to make sure you find an engine you like working with ahead of time that doesn't pigeon-hole you into a game type, so you can be flexible for the jam theme but also not have to learn it all in the very short jam window. I spend a lot of my time just thinking like "Man I like this game, I wanna try making my own version" and practice in engines like Godot.
For me with programming, it's a lot about building your mental toolbox and learning how to break down concepts into the logical flow so you can do that in your game. What I found that helps me the most is practicing taking whole ideas and then thinking about what do I need to make it all work and writing it out in order. You'll only ever execute one line of code at a time, no matter what, so if you think of your game projects in broad strokes like:
- Get input
- Update game
- Update player
- Change from idle to walking, move around, etc
- Update enemies
- Perform AI logic
- Update GUI
- Check the player info and update what to draw later
- Update player
- Draw game
- Draw player
- Draw enemies
- Draw GUI
You can then just focus on small portions at a time and even inform what to google in a more focused way or decide what order things need to happen in to get the final effects you want. Hopefully this helps you or anyone else reading this! I'm by no means an expert, but this is my method and it helps me stay more focused especially when my ADD acts up in a project and i get bored and jump around working on different components lol.