Hey, we ALL felt like this at that age. So many exciting things to do, so little time! (And if you are an adult with a family, it only gets worse! 
)
First, Visual Studio Unreal Engine uses is free, so that’s sorted. 
Secondly, you are trying to become a programmer, animator, modeller, writer, artist and designer, at the same time! That’s enough to give anyone brain-ache, so don’t feel hard on yourself. Take short-cuts. That is what they are there for. Blueprints are great for prototyping, as they allow you to build something quickly, and redo it again and again, until you are happy with it. Once that is done, you can rewrite it in c++ to improve performance. Mixamo’s Fuse, Autodesk’s Character Generator or MakeHuman’s MakeHuman are great ways to generate characters, which you can always replace later. These things are there to help you build something in the first place, and you can polish them as much as you like later. Triple-A graphics are the least important thing you need to think about when you are first building a level. Don’t get hung up on this when you are just experimenting.
One piece of advice I always give people, is to break things up into smaller pieces. So you want to make an open-world?
First, learn how to make a character, and get them running around a level. (Or first person movement, if that is what you want).
Second, learn to build landscapes or buildings. (As an advanced subject, learn to generate them procedurally).
Third, learn level streaming, so the level is bigger than can be held in the computer’s memory at one time.
If you learn this, you suddenly have the basis for an open world level. Yay! (Although this isn’t to trivialise the amount of learning this will take).
Make no mistake, some of this stuff is hard, but keep watching YouTube tutorials, especially Unreal’s official ones. See what can be done in the engine, and how to approach things. Seriously, watch their YouTube videos.
Look on the bright side, you’re 14, with a brain like a sponge, and you seem to suit this kind of thing, with interests in mathematics and sciency stuff. You couldn’t ask for a better start.
Good luck!