Envy for your past and dealing with letting go

I understand that I will look back on where I am in four years and hopefully look back on it fondly. I think this is the case because right now, as I type I reminisce on life four years ago. However, in the future when I look back, I will think of different things than I do when I think about my past right now. When I am an adult and have exponentially more responsibilities than I do now, I will look back and remember it as a fun time when I didn't have to do anything. My house and my food were both paid for. I didn't have to worry about paying for my meals or working for my money, all I understood was video games and computers. At most all I worried about was my grades, but that changes when you leave school. This leads me to draw the conclusion that I should live in the moment and not take right now for granted. This brings up two things, however. One of the things is, how do I know that I am living the way I want to remember in four years. The second one is, how am I supposed to "live in the moment" when my future is so insistent on being planned and my past is begging me to not let it go. I'm constantly plagued with thoughts of how it could be better if X didn't happen. Or X sucks now because Y happened instead of Z. Oh well, my older brother brings up a good point. He says "Let your soul persevere and let your demons be demons" and he explains that this means that the "demons" are my past and future and that I should just ignore them and let them do whatever while I let my soul -- my mental health and wellbeing -- flourish without them bringing it down.

Oh well, to anyone who decided to read. Thanks, this isn't for anyone in particular just had to get thoughts off of my chest. I'm Ewan, I'm a freshman in high school who hasn't been doing well the last few months.