You being young will actually change my advice a little bit. I assumed you were a middle aged man looking to upgrade, but assessors (and I) tend to judge young guys differently.
Real quick, if you set up a "us soccer game officials" account you can view your assessment feedback and scores. I think US Soccer is changing their platform soon, but still get gameofficials in the meantime.
Also, if you are young, try to get in and do DA (Developmental Academy) games. The league is expanding a bit and KM is looking for young new referees to get into the program. You will get the U12 solo Centers and AR's for U14-15 for the first year. As long as you seek him out, introduce yourself, look young and fit, and are willing, KM will give you a shot.... I think.
Fitness is a given for young guys. If you are young and not fit, then what do you have? For high level assessments, getting all the basic foul recognition is the floor. You don't get points for not screwing up in that department, it is a given, a baseline requirement.
Last thing in this post that I will say: educate yourself. The referee education system by itself is not enough. Become a nerd about being a referee, you have to love it that much to shoot up past your peers. Take a day or two, buy the physical Laws of the Game book, get a highlighter, pen, notebook, and post it notes and read that sucker cover to cover. You will notice a few sentences there that start with "A referee must...". Those are the non-negotiable decisions. You probably thought you could manage a Stopping a Promising Attack (SPA) foul without a card, but that is one of those "must" cards. Read from the site
http://asktheref.com/ regularly. When you have time, just go back in the archives and read the responses until 2008. There is a really funny post around 2008 if I remember. Next, subscribe to MLS YouTube channel and find all the video's with "Instant Replay" in the channel. Click on the playlist, press the mute button (important), and watch the archives. That video series is essentially a highlight reel of difficult calls that referees have to make. And go the EVERY RPD. Not just 3, not just the minimum 5. Every one that you can. Then go to the association meetings, and go to the symposium in the fall even though you have enough hours. Don't be a mute, introduce yourself to many different people. Let them know you are excited about refereeing and are trying to get higher level games. Tell this even to your peers that would have no control on what games you get. Ask for feedback in every game. Take it with a grain of salt. Self assess yourself every game. What did I do? How did it work? Would I do it again? Why were people mad? Is there something extra I could have done to make that not so even though I got the call right?
If you do all this, your love for and knowledge of the game will bleed through and change your perception subconsciously on you actual assessments. Also, treat every game like an assessment. If this site teaches you nothing else, let it teach you that referees love to talk and gossip about each other.