I echo everything Cyangmou has said.
It's also a lot about time spent. How many hours a day can you devote to this? And can you also be mindful of how you're spending your time?
In my opinion, art needs to be one, if not the most important things in your life, if you want to become good at it. There's a reason why artists often forget to eat/sleep/etc. I am not encouraging unhealthy behaviour, but to become truly skilled at anything, with finite time and energy, there is simple math in how you are dividing your efforts.
If you're starting from almost nothing, you could potentially get as proficient as the example in a few weeks, but you need to engineer your life to meet what that demands.
I recommend the following:
-Plot out how much time per day you have not obligated to other things.
-Drink lots of water. Your brain is 75% water. Also stretch often to improve blood flow. Your brain is the organ responsible for art.
-Detach your ego from your work, so that you can share it for critique without fear. In general, practice detachment. You can feel pride for your accomplishment, seeing improvement, etc. But to start defending singular pieces, or empathizing with them strongly enough where you dwell on them, is against the nature of growth and development.
-Recognize you don't know what you don't know, and that you will always underestimate the effort or skill something takes to achieve.
-Use most of your time to draw. What do you want to represent? You like JRPG type stuff, it seems, so draw humans (real humans, actually study humans, because if you like anime, those drawings you like are utilitarian means to communicate and the artists studied humans to be able to draw them) and trees and medieval houses and monsters/animals. Anything you want to pixel, draw it on paper.
-Use a smaller amount of time creating assets. Post them to the forum for critique, but also try to move on when you notice diminishing returns. Sometimes it takes a new angle of looking at things, to come back to something and do it better than you once did.
-Replace much of your entertainment/tv/gaming etc with watching other artists teach or draw/paint. Absorb.
-Nourish your creativity by following other artists, or sources of inspiration like nature photography, architecture, etc.
-If you have 25-100 dollars or so, buy an entry level graphics tablet off amazon, if you don't have one already. Pixel art with a mouse is like trying to draw by holding a shoe with a pencil stuck through the bottom.
If you can spend 8-12 hours every day, for 14 days, you could become as good as that example. The example you posted would be considered beginner level.