There are many mistakes you can make as a developer throughout your career. Some you will not realize until later and you will wish you already knew them. I'm not talking about programming languages or frameworks or anything technical, this is more related to your work ethic and how you handle yourself throughout your years of experience.
Know Your Worth
I put this at the top of the list because this is, in my opinion, the most important thing you need to learn. This is especially important if you're coming across a new opportunity. Sometimes the opportunity is less than what you deserve, but you still consider it thinking it's the only option. Other times it could be a great opportunity, but you think you don't have what it takes to do it and you back out from trying.
It's not just in new opportunities. Some people get stuck at a job that isn't help them advance or learn anything new just because they think they won't find a better opportunity out there.
You need to believe in yourself and know your worth. It's not about how many technical stuff you know. Yes, that part is obviously important as well, but you can always learn new languages or frameworks or concepts. If you find yourself lacking in this part and that's standing in your way, then start advancing and learning so that you can keep valuing yourself and aiming higher.
Value Your Years of Experience
This doesn't just apply to those that already have more than 2 years of experience, this also applies to anyone just starting out now. Value the time and effort that you have put or are putting into a job or a project, regardless of how big or small it is. Every day through working on it, you gain and learn something new, which makes future experiences easier.
This mostly happens with freelancers. A lot of freelancers are unsure of how much they should price their work, scared that it will be too high. Same thing for full time developers that are scared to ask for a raise or leave a job that's underpaying them. Remember how much time and effort you've put into getting to where you are now. When you work with or for someone, you are bringing your knowledge and experiences with you, and that's worth a lot. So, don't be scared of asking for exactly your worth. You are working hard to become better and deliver high quality services.
Advance Your Skills in A Specific Path
When you become a developer, you may get overwhelmed by all the things you can learn and do. You might dip your feet a little in Web Development, a little in App Development, Game Development, etc... It's nice and important to learn a little of everything of course, but make sure that you pick one path that you like the best and focus on expanding in it. Technology is always advancing and growing and you need to be at the top of it. If you don't choose one path, you might get lost in the chaos and end up an expert in nothing. Focus your attention and energy to one of them and become the expert in it.
You Don't Need To Know Everything
This is a follow up to the previous one. You might get intimidated by others who know more than you or think that you need to know everything to be good at this job. You don't. You need to remember first and foremost that no one knows everything, and there will always be someone who knows more than you, just like you'll always know more than someone else. Keep focusing on the path you have chosen and moving forward in it. Let other people's knowledge inspire you, but not intimidate you or distract you from your own path.
Everyone Uses Google
This is more directed towards beginners or those that are just learning about development. If you run into a problem or forget something, don't be embarrassed or think it makes you less to use Google. Everyone, including those with many years of experience, can forget sometimes and have to google a syntax or a function or an error they face. There's nothing wrong with it and in fact you need to learn how to use Google to your side. Wracking your brain and wasting time trying to figure it out yourself is not always desired, especially when someone out there have already went through it and you can learn from it just by searching it.
Don't Let What Looks Complicated Discourage You
I think we all have went through this or some are going through it now. You see something that looks complicated, maybe above your level, and you think there's no way that I'll be able to do this. Whatever it is that looks complicated or hard to you, know that if you give it time and you work hard on yourself, with time you will realize that it isn't.
Don't Look Around, Keep Looking Forward
This one took me a while to understand and apply. As a woman in this career, I have faced some problems finding a job and dealing with clients, who undermined me just because of the fact that I'm a woman. It can get discouraging and make you want to give up, but the best counter attack is to keep working hard on yourself. If you feel like you are judged in any way in your work place or when finding a job, don't let that put you down. Instead, work hard on improving your skills, whether they are soft skills or technical skills, because with time you will become an asset that they can't look down on.
You should also remember that at the end of the day it doesn't matter how others look at you, what matters is how you see yourself. Work hard to be confident in your own identity and not let anyone put you down by their judgments.
These are some things I've learned through my journey as a developer. If you have other lessons that you think others should know as well, please let us know in the comments!