It's hard. It's really hard.
At about 19 I started racing cross and decided I wanted to give 'going pro' a try (I raced MTB as a junior starting at ~13, but not very seriously). After a few years of training and rising through the ranks, it finally got to the point where I had entries paid for and deals on kit, but was in no place to quit the day job. Fast forward a couple years, I'm riding for a small team. They pay for me to get to the races and I have full gear sponsorship, but income is still laughable. So at 25 I was at the point where I was riding to podiums in state (CA) champs and top 20s at national champs. It was about then I looked around and asked myself, "what am I doing?"
I realized that the pecking order was pretty well established. There were two types in front of me: older, well established riders, or up and comers. They had all been at the top of the game throughout the junior and U23 ranks, and it was just a matter of generational exchange. Maybe I could have spent a couple of years moving up another 5-10 places in the big races, but as soon as I'd gotten there the sun would have been setting on my peak racing abilities. Luckily I'd been investing in my brain at the same time, and I had a nice contingency plan in place!
This is in cycloross where the fields (and pockets) aren't too deep, and domestic cyclocross at that. That said, I could imagine that in road, where 99% of what matters is how big your engine is, it's even harder to break in relative to the kids that have been training since age 9. And the money isn't guaranteed either - the current US road champion, Eric Marcotte, has a day job as a chiropractor. A good friend of mine (former Collegiate Div1 national champ on the road) once confided that he expects less than 5 US pros have ever been able to retire on their cycling salary+winnings alone. All of these were racing and destroying the senior competition as juniors.
Best of luck, and enjoy riding! If I were you, I'd expect to be a Cat 2 by the end of the year if you want to try the pro ranks with any seriousness. If that doesn't happen, I would seriously consider alternatives!