1. You have bad habits
The only way to prevent poor technique is to be observed and instructed by someone that knows what to look for. Bad technique habits can and already are affecting your sound quality, playing endurance, and rhythmic consistency.
2. You’re missing important concepts
What you already know is a powerful learning tool, but only if an instructor is there to show you the next step. A lesson teacher’s explanation of how your experience and ability relates to new skills will make learning much easier.
3. You need motivation
You might not need motivation to play, but you need motivation to learn. Someone needs to help you focus on the things you can’t do yet. Without private lessons your skill set will grow only to a certain point – and then stop.