By really internalizing the fact that you're still learning. You just started. You do not have to be at the same level as the guy who's been selling paintings for fifteen years now. It's an open door, but you have to aware of this. A lot of people say things like 'some are born with it, but I had to work for it' when the truth is that we all have to work for it. Hard. And a lot. And the inevitable result of this is that it's going to take a while to get at the level where you want to be. But that's okay. No artist is ever done learning.
Be patient and forgiving with yourself. You are learning. You don't have to run a marathon in under five minutes. As Susan said; try to find inspiration in the works you admire. Right now they seem intimidating, but try to figure out how they do it. See if you can find work in progress images. Learn from them. Find out what makes that work so impressive. And then break it down into little things that you can work on.
They once were at the very same place where you are now, looking up to others and probably feeling insecure as well. Hell, maybe they still are.
Everyone does things at their own pace. The people your age who are already a lot further may have been at it for years now, may have had teachers around them that helped them improve faster by nudging them in the right directions; there are lots of factors that influence how quickly you improve.
Overall; as hard as it may seem, try not to sweat it too much. Allow yourself the time to grow and learn from people who have already learned what you wish to learn. The insecurity might not even ever go away, but that's alright as well. We all have our insecurities, but as long as we do not allow them to drag us down and continue working on them, it'll be alright.