Thank you! Sadly the movement wasn't really super well explained in the game, but you can actually slow down with Shift on the keyboard. It is also possible to move the character more precisely with the analog stick on a controller. Although ironically, the only bosses where I felt like going slow was really necessary were Rust and the final one.
As for jumping, the falling speed is actually very fast. I think you might have been either holding jump or shooting the whole time, as those things keep you in the air.
Regardless, thank you for giving it a shot! If you beat Rust, you were actually really close to the end of the game. :)