Growing up, you were told what you could and couldn't do, say, wear, and more. However, when you get to be an adult, you don't have to listen to that anymore. You are an adult who is at the age and (hopefully) the intelligence to make decisions for yourself. Why would you continue to allow people in your life to make your decisions for you? That is an absolute disservice to yourself as an individual.
Yes, it can be scary to stand up for yourself but sometimes it needs to be done. If you speaking your mind is what stands in the way of you achieving a huge life milestone, why wouldn't you do that? So what if your best friend doesn't like that you want to buy that specific car? If you can afford that car, why would you let their opinion stop you? So what if your parent doesn't like your major? If you are happy with what you want to study, it shouldn't matter. A big thing to remember is that you can absolutely stand up for yourself and your future while being respectful.
Something I personally have seen recently is that people either go full blown angry when they stand up for themselves or they don't say anything and continue to get walked all over. You need to speak up for yourself before it gets to your personal breaking point. The best way to do it is sitting down and really thinking through what you want to express to someone, the pros and cons of the thing you want to do, and so on. Remember that you shouldn't expect anyone to fully support what you do. The only thing that they should be doing is respecting you as their loved one and as a human being.
Standing up for yourself and doing things to better yourself despite what people may think is one of the biggest steps to being an adult. Not to mention that nine times out of ten, people really don't care about what you're doing. It's easy to think that everyone is looking at you but I promise that most people have better things to do than focus on what you're up to constantly. That was a lesson that took me quite a long time to understand, especially considering that I have anxiety. That being said, if you haven't been told this lately, I believe in your ability to do what's best for you. I believe in you to work hard to afford the things you want. You are brave and intelligent and hard working.
