Congrats on your first game ever! It should not be the last. This is a sweet little platformer, quite difficult and unforgiving as the genre dictates :) Really love your graphics, and yes I recognize the source but you put it to good use!
Fine-tuning a platformer to make the controls satisfactory is really hard. It is important to expose many parameters (delays, jump power, collider margin of error, speed in all directions, all that) and keep tweaking them ever-so-slightly until you are happy. The masters of the genre used months getting their games right! Yours is absolutely not bad.