ngl dude, like 99% of the population will feel unfulfilled and sort of aimless when their 18. Anybody that says otherwise is lying or they are some super disciplined prodigy who's had their life mapped out since they were 5, and are very much not ordinary. The sooner you let go of the idea you didn't make the most use of your time in the past, the quicker you can get to the opportunity to make the most of your life in the present. Also, considering you're 18 that probably means you're going to college, and if that's the case, make the most of that time; it's probably the last phase of your life you won't totally be swamped with other responsibilities like juggling a job, your family, and your bills, so make the most of it.