Logo design is a very particular skill, so don't beat yourself up over that one. Similarly, icon design is pretty tough too. Probably don't try to start there.
For a dev there are some really high-value principles that aren't too hard to learn but hugely improve your ability to understand design:
Maybe for your next project, try focusing on colour and typography first; then add much more white space than you usually do; and work out which pieces of content should be closer together (proximity). You'll be surprised how much better you can make things look just by 'letting them breathe'.