Administrator issue

About 3 years ago I purchased my MacBook Air brand new from Apple and I have been the only person to ever have an account on it. For the past year or so, I've noticed that anytime I try to make any system changes I have to provide an administrator username and password, however, mine do not work, and when I look under "Users & Groups", my account is just standard and the box that says "allow user to administrate this computer" is not checked. This is becoming too much of an issue now, I can't update software and I can't install things I need. I can't remember whether my account details used to work as an administrator or not when I first bought it, but for at least the past year (most likely longer), they have not. I'm the only person who has ever used this computer and there are no other accounts on there, so I cannot understand how I am not the administrator. Someone please help, this has been going on way too long and I have no idea what to do anymore!