I'll be lying if I said I knew how long a journey it'll be to learn to play an instrument. Apart from the tough pieces, difficult passages and the most difficult lessons I had to face few of the most important lessons were left untouched.
You should be your best critic.
We all love to believe that we've mastered that boring lesson because we want to move on to the next one and over with it. If you are a self taught musician then this lessons is more important than probably anything else. Even if you are learning with a good mentor, the chances are they'll review your passages in a test or two. It's very likely that in a perfect situation you can actually pull off that passage but it's only you who knows deep down how much effort you require to pull it off and what endurance level you are on. This one thing is very important. The smallest of the lesson fit into a bigger picture later. So we should be brutally honest with ourselves first and be true towards the lessons.
Slower is more difficult.
Let's face it we all want to go into Lamborghini mode and play fast as soon as we can. Many a times you'll actually feel comfortable doing that too. But have you actually mastered that lesson? The only way to find out is to go slow on it first. Playing slow needs patience and your sense of rhythm needs to be very strong. It's easier to overlook timing errors at fast pace but at slow pace even the slightest loss in sync is clearly audible.
So always use a metronome, start slow and build up the speed. Again refer lesson 1 to know when to speed up.
Practicing wrong techniques is worse than no practice.
I'm a self taught guitarist and my approach to guitar initially was pretty laid back. I thought what feels natural must be the right way to go. Well technicality it's not always true. I practiced with wrong picking motion for a couple of years and when I began building up speed, I hit a roadblock pretty soon. There was no going forward. It took me another year to change my picking motion and technique and it was one of the hardest things I had to do. Unlearning and relearning is the most time consuming and verbose task you can ever do.
Always start the learning with a good and well tuned gear.
Most of the people who take up music as a hobby or even professionally end up buying a cheaper alternative as they don't want to damage expensive gear during practicing. This is one of the biggest mistakes you can make. A cheap alternative gear is not very reliable when it comes to intonation. The degree of inaccuracy in sound is higher. This essentially means that though your muscle memory is building over the time by practicing on that instrument, your ears are training themselves to wrong reference notes. The music that we learn in the initial years train our ears and lay the cornerstone of our ear training. When you'll actually move forward and go to a better intonated gear, it'll rather sound unpleasant to you. See the damage?
Unlearning and relearning is a pain.
Not following your heart.
Every person has a preference when it comes to the sound, playability and the feel of an instrument. Choosing an instrument just because someone said it's cooler, or someone asked you to do it is highly likely to guide you into a rut very soon. Identify which instrument you like the most and take that up and learning will never be a pain.
So fill yourself with passion, have no inhibitions and practice like you are going to perform in your dream band tomorrow!