I got stuck as well but saw the comment you made about shift to run. Made it to the boss and almost beat him I think lol
It's a fun little platformer game, with fun graphics and solid music and sound. Good job! I do think you have a (big) memory leak somewhere or you're doing a lot of unnecessary calculations. My frame rate got progressively worse as I went and was basically a crawl by the time I was fighting the boss (whole game in slow motion). After I died, the frame rate went back to normal.