Music is best learned through mentorship, so taking private lessons is highly recommended. It's not easy evaluating yourself. In fact, critiquing your own skills can be a confusing task. Your perceptions are always biased one way or another. This is where a teacher comes in.
Every bassist should take at least a handful of lessons. If you're serious about playing this instrument, it's recommended that you find a qualified teacher and work under his tutelage for months or even years. As you become more advanced, you should study with a variety of teachers. To do this, you may need to travel to different cities or countries. This is also recommended since you may not have all the resources you need in your hometown.
Music lessons are not just for kids and amateurs. Professional musicians often take lessons from time to time as part of their continuing education. Music instruction doesn't end with a college diploma or a successful performing career. The quest for knowledge and growth should continue indefinitely.
The best musicians are humble about what they know. This is because they realize that there is always something to learn. It's best to always consider yourself a student. Even if you're a professional, it's important to remain open to change and growth. It's arrogant to think that you know absolutely everything.
Self-taught musicians sometimes claim that they don't take lessons because they think a teacher will spoil their individuality or crush their creative spirit. This is an immature attitude and one that will limit your progress. As a student, you may find that there is some advice you reject from your teacher. This is okay, as long as you don't do it out of ignorance. You will stunt your musical growth if you completely reject private instruction because you think that teachers ruin individuality.
Good teachers help you to learn faster and they help you to develop your own style. A teacher will also unlock many mysteries for you. This will allow you to develop at a quicker rate, and you won't spend as much time reinventing the wheel.
Further, if you work with a teacher, you will be able to bounce ideas off of one another, and this alone is invaluable. Most of all, a teacher is a reliable and constructive critic who will help to shape you into a confident musician. Learning to play any instrument can be difficult. Why not let someone help you? It will make the journey a lot less bumpy.