You shouldn't give up on music composition. Why would you if it's who you are? Are you giving up on yourself? Who cares how much knowledge someone else has, there will always be someone who knows more than you about music. That in no way should deter you from really living if you have found who you are as a person.
When I made the decision for music to be my career, my dad was pretty disappointed. He didn't talk to me for a while, and when he finally did, he only told me that artists will suffer. I knew he was right, and I knew he said that because I excelled in school and could have chosen an easier path to success. But I knew who I was, and only you really know that.
Of course it absolutely helps to have a leg up with tutors. I teach private guitar/piano lessons for a living (I would be performing for supplemental income, but well, you know, Covid in the U.S..................), and sometimes I'm just amazed at what they're learning at early ages. But a lot of them are ungrateful rich kids, and don't have the passion you have. And honestly, I think the passion for composition is the only fuel you could have to move forward, no matter where you're at with your music education.
Go for something unique in your writing. Be you. Sure you could post another pastiche form that composers 200 years ago did way better than you might ever do, you could adhere to a common practice harmony and study Bach all your life. You might attain a position at a university, teaching the same shiit that a million others do when you could just buy an 18th century counterpoint book. Sure, learn from that, but keep going forward. The evolution of music stems from hearing what the world around you has done and is doing, and putting the Keaton Hoy spin on it.
The thing is, you'll never know it all, and neither will the ones who had an advantage over you with earlier lessons. You've completed AP theory, I'd say that's a one up over some people who are applying against you. I'd focus on finding your musical voice, absorbing all you've learned and molding it into what you want to give this world. Keep in mind, we live in the information age. It's 2020, dude. I don't know what kind of music you write, but cater it towards the age in which you live and I guarantee with perseverance and lots and lots and LOTS of HARD WORK, you will find success as a composer.
No one is going to copy your work. And plus, when you post a written or audio form of your music online, it is automatically a copyright. I'd say post your music for peer review if you're trying to put together a portfolio. There are tons of wonderful people here that give great reviews and constructive insight to members works FOR FREE. Take advantage of that! It's truly a humble community, and being here off and on for years has helped keep my inspiration alive.
Oh, and welcome! 😄