The good news is this is not a time essay. Go through the brainstorming exercises from college essay guy. Once you put something (anything) on paper, it will be easier, even if ultimately you end up with something completely different. If you have thoughts and ideas at unexpected times, jot them down quickly on your phone so you can refer back.
Again, while you should take it seriously, it does not have to “make” your application. Make sure it is proof read many times over and grammar is perfect. Be proactive as about feedback from teachers. Good college essays take a lot of tweaking. It is not a sit down and get it done thing, but you need to have something to work with first.