You totally are an expert, in your own way.
You have to condition your mind to believe that, and then your work will reflect that.
At the same time, you must actually study your niche and be a "high information person" to have enough data to feel adept in your area.
Now a days, you don't have to wait for some certificate to tell you you're an expert, you position yourself as such.
Because we rely on SOCIAL PROOF to confirm the validity of something, a good way to use this is to put yourself out there A LOT with videos, posts and sometimes even controversy, to get people talking about you.
When people KNOW WHO YOU ARE, that's enough social proof right there.
Most experts get a ton of bad feedback and haters. This is how you know you're on the right track. It has been said that if you're extremely effective at something, 95% of people will criticize you for it.
You are the expert. Believe it and make it real.